diff --git a/core/sheerka.py b/core/sheerka.py
index e9fc95e..09a5a2b 100644
--- a/core/sheerka.py
+++ b/core/sheerka.py
@@ -85,7 +85,7 @@ class Sheerka(Concept, metaclass=Singleton):
         return ReturnValue(True, self.get_concept(Sheerka.SUCCESS_CONCEPT_NAME, True))
 
     def eval(self, text):
-        #evt_digest = self.sdp.save_event(Event(text))
+        # evt_digest = self.sdp.save_event(Event(text))
         result = self.try_parse(text)
 
         return_values = []
diff --git a/docs/blog.rst b/docs/blog.rst
new file mode 100644
index 0000000..641bfa8
--- /dev/null
+++ b/docs/blog.rst
@@ -0,0 +1,228 @@
+2019-10-30
+**********
+
+What is Sheerka ?
+"""""""""""""""""
+
+Sheerka is a *communication* language,
+as opposed to the traditional *programming* languages. Its
+purpose is to ease the communication between the (wo)man  and the machine,
+ultimately using the voice. I will first use it to program faster, and maybe
+more easily.
+
+.. _ulysse31: https://fr.wikipedia.org/wiki/Ulysse_31
+
+Where does the name Sheerka came from ?
+"""""""""""""""""""""""""""""""""""""""
+Sheerka is my misspell of Shyrka, from my childhood anime ulysse31_.
+For those you don't know this old cartoon, it's the Odyssey story from Homer,
+ported in the 31st century. Ulysses has a spacecraft with an AI named Shyrka
+
+I was a great fan of this cartoon when I was young. I thought that the idea of
+bringing the ancient story of Ulysses in the future was a bright.
+
+Ever since then, Sheerka was my reference for any sophisticated computer. Unfortunately
+for me, at that time there was no wikipedia to tell the the correct spelling.
+
+Model v0
+""""""""
+In my view, the beginning of everything  are the **Events**. Basically, they are the commands (ie requests)
+entered by the users.
+
+The events are parsed, to understand what is required, so they produce a new **State**.
+The state is a like a big dictionary that holds everything that is known by the system.
+
+Most of the elements saved in the **State** are the **Concepts**. In this first version,
+it's a little bit complicated to define what is the **Concept** as it can have several
+usages. To make it simple, I will say that a **Concept** is an idea that can be
+manipulated by the rest of the system.
+I am pretty sure that its form and usage will evolve as I will manipulate
+them
+
+- Each **State** has a reference to the event(s) that trigger this state
+- Each **State** has an **history**
+- Each **Concept** has an **history**
+
+
+An **history** is a triplet of
+
+- user name
+- modification date
+- digest of the parent
+
+.. _git: https://git-scm.com/
+
+Personally, i have taken this way of tracking modification from how it's done on git_,
+I guess Linux Torvarlds took it from somewhere.
+
+
+2019-10-31
+**********
+
+More on Concepts
+""""""""""""""""
+To define a new concept
+
+::
+
+    def concept hello a as "hello" + a
+
+
+Note that the traditional quotes that would surround hello a are not necessary.
+In this example 'a' is a variable, as it appears as variable in the 'as' section
+
+So, you could call the concept by
+
+::
+
+    hello kodjo
+    hello my friend
+
+They will produce the strings "hello kodjo" or "hello my friend"
+
+About versioning
+""""""""""""""""
+As I said previously, I mimic how git_ versions its objects.
+
+::
+
+   Obj v0 :  parents = []
+            user name = <a name>
+            modification date = <a date>
+            digest = xxxxx
+
+    Obj v1: parents = [xxxxx]
+            user name = <a name>
+            modification date = <a date>
+            digest = yyyyy
+
+    Obj v1: parents = [yyyyy]
+            user name = <a name>
+            modification date = <a date>
+            digest = zzzzz
+
+I always keep a reference to the last version of the object, so I can navigate through
+the versions using the :code:`parents` attribute of the object
+
+In git_, there are basically two types of objects :
+
+- **content** (file content, or directory structure)
+- **reference** to content (commit or tags)
+
+The hash a **content** only depends on it, while the hash of a **reference** also depends
+on the user name, the modification date and the parents. In both cases, the hash is
+computed on the whole object. So the hash can also be used to check the integrity
+of an object.
+
+For my objects, I need to decide how I compute the hash.
+
+**Concepts** have history, if I decide to include the history in the hash,
+as the modification date is :code:`datetime.now()`, a new version will be created
+even if the **Concept** has not changed. If I don't include it, the integrity of the
+what is saved is no longer guaranteed.
+
+I choose to value identity over integrity. The hash code of the **Concepts** does not depend
+on his history. We will see what the future will say about this.
+
+2019-11-01
+**********
+
+Inspired by CodinGames
+""""""""""""""""""""""
+
+
+.. _codingame: https://www.codingame.com/home
+
+I am trying to teach my little kid how to code. He is 12 years old and it was his very
+first time.
+
+Rather than trying a standard formal approach, we went on the codingame_ web site. There
+are some pro and cons to use this platform, specially for the very beginners, but
+I like the visual output of the programs . It's really like coding a game !
+
+What I haven't noticed previously, is that (at least for the first programs), the solution
+is given in human language.
+
+For example, for the exercise called "The descent" you will find
+
+::
+
+    For each round of play :
+        Reset the variables containing the index of the highest mountain and its height to 0
+        For each mountain index (from 0 to 7 included) :
+            Read the height of the mountain (variable :code:`mountainH`) from stdin
+            If it's higher than the highest known mountain, save its index and height
+        Returns the index of the highest mountain on stdout
+
+It will be great if Sheerka is able to produce some code from these instructions.
+
+Some words data persistence
+"""""""""""""""""""""""""""""""""""""""""
+As I previously said (or not), the main difference between Sheerka and other languages,
+is that Sheerka has a memory of its (her ? :-) previous interactions with the users.
+
+The **Concepts**, as well as the **Events** or the **Rules** are persisted. Because of
+that, I think that the more Sheerka is used, the more easier it will be to use it.
+
+So my first focus was to decide which database to use.
+
+There are tons od different databases already on the market. Unfortunately for me, I'm not
+a database expert. But, I already knew that I was not looking for a traditional
+relational database (SGDB) as the structure will evolve and I didn't want to spend
+my time on redesigning the schemas and the constraints.
+
+As I was learning Python, it could have been a good idea to also start looking at an
+already existing NoSql database. I started to look at MongoDB, but I got lazy. I knew that
+the top feature was that management of the history (the way git does it), and it was not
+provided by Mongo, or I didn't notice it in my first readings on the subject.
+
+So I decided to design and implement my own database.
+
+
+SheerkaDataProvider (sdp)
+"""""""""""""""""""""""""
+Not I great name, I confess. But who care ?
+
+What are the main design constraints?
+
+::
+
+    1. No adherence with the filesystem.
+        We must not care about where the data are stored.
+        The first implementation will be file based, but it has to be extensible.
+        The final target will be to have a decentralized persistence
+    2. CRUD operations are designed according to my needs
+        I don't want standard CRUD operations that will be tweaked.
+        The direct consequence is that this library won't fit any other purpose
+    3. History management for State and other objects for free.
+
+
+sdp, like many modern database systems, is a dictionary. A big list of key-value pairs.
+The key is a string, the value can be almost anything. Actually, for my needs, I guess
+that I only need strings, numbers and list (of strings and numbers :-)
+
+I need one level of categorization. That means that my objects can be group. The basic
+signature to add a new element :code:`add(entry, obj)`.
+
+with
+
+::
+
+    entry : is the group / category where I want to put the object
+    object : object to persist
+
+With :code:`add("Concepts", "foo")` the database, let's call it **State** once for all, will be updated like this:
+
+.. code::python
+
+{"Concepts" : foo}
+
+If I want to have another entry, I don't want to care about what was previously done. I
+need the second call :code:`add("Concepts", "bar")` to produce
+
+.. code-block::json
+
+{"Concepts" : ["foo", "bar"]}
+
+So we are no longer in the usual way of implementing a CRUD.
+
diff --git a/sdp/sheerkaDataProvider.py b/sdp/sheerkaDataProvider.py
index be47436..259baf4 100644
--- a/sdp/sheerkaDataProvider.py
+++ b/sdp/sheerkaDataProvider.py
@@ -49,6 +49,18 @@ class Event(object):
         self.message = as_dict["message"]
 
 
+class ObjWithKey:
+    """
+    Internal key value class to hold the key (and the value)
+    when it is detected
+    It's created to distinguish from {key, value}
+    """
+
+    def __init__(self, key, obj):
+        self.key = key
+        self.obj = obj
+
+
 class State:
     """
     Class that represents the state of the system (dictionary of all known entries)
@@ -62,25 +74,73 @@ class State:
         self.data = {}
 
     def update(self, entry, obj, append=True):
-        obj_to_use = {str(obj.get_key()): obj} if hasattr(obj, "get_key") else obj
+        """
+        adds obj to entry
+        :param entry:
+        :param obj:
+        :param append: if True, ducplicate keys will create lists
+        :return:
+        """
+        obj_to_use = {obj.key: obj.obj} if isinstance(obj, ObjWithKey) else obj
 
         if entry not in self.data:
             self.data[entry] = obj_to_use
-        elif isinstance(obj_to_use, dict):
-            if append:
+
+        elif not append:
+            if isinstance(obj_to_use, dict):
                 self.data[entry].update(obj_to_use)
             else:
                 self.data[entry] = obj_to_use
+
         elif isinstance(self.data[entry], list):
-            if append:
-                self.data[entry].append(obj_to_use)
-            else:
-                self.data[entry] = obj_to_use
+            self.data[entry].append(obj.obj if isinstance(obj, ObjWithKey) else obj)  # do not use obj_to_use !
+
+        elif isinstance(obj_to_use, dict):
+            for k in obj_to_use:
+                if k not in self.data[entry]:
+                    self.data[entry][k] = obj_to_use[k]
+                elif isinstance(self.data[entry][k], list):
+                    self.data[entry][k].append(obj_to_use[k])
+                else:
+                    self.data[entry][k] = [self.data[entry][k], obj_to_use[k]]
+
+        elif isinstance(self.data[entry], dict):
+            raise SheerkaDataProviderError(f"Cannot found key on '{obj}' while all other elements have.", obj)
+
         else:
-            if append:
-                self.data[entry] = [self.data[entry], obj_to_use]
-            else:
-                self.data[entry] = obj_to_use
+            self.data[entry] = [self.data[entry], obj_to_use]
+
+    def modify(self, entry, key, obj, obj_key):
+        # if the key changes, make sure to remove the previous entry
+        append = False
+        if obj_key != key:
+            self.remove(entry, lambda k, o: k == key)  # modify from on object to another
+            append = True
+
+        self.update(entry, ObjWithKey(obj_key, obj), append=append)
+
+    def modify_in_list(self, entry, key, obj, obj_key, obj_origin, load_ref_if_needed, save_ref_if_needed):
+        found = False
+        to_remove = None
+        for i in range(len(self.data[entry][key])):
+            item, is_ref = load_ref_if_needed(self.data[entry][key][i])
+            if not hasattr(item, "get_digest"):
+                continue
+            if item.get_digest() == obj_origin:
+                obj = save_ref_if_needed(is_ref, obj)
+                if obj_key == key:
+                    self.data[entry][key][i] = obj
+                else:
+                    to_remove = i
+                    self.update(entry, ObjWithKey(obj_key, obj), append=True)
+                found = True
+                break
+
+        if not found:
+            raise (SheerkaDataProviderError(f"Cannot modify '{entry}.{key}'. Item '{obj_origin}' not found.", obj))
+
+        if to_remove is not None:
+            del self.data[entry][key][to_remove]
 
     def remove(self, entry, filter):
         if filter is None:
@@ -108,13 +168,29 @@ class State:
         return hashlib.sha256(as_json.encode("utf-8")).hexdigest()
 
     def contains(self, entry, key):
+        """
+        if key is None, returns True if entry exists
+        if key has a value
+            returns True if entry is an dict and contains key
+        :param entry:
+        :param key:
+        :return:
+        """
         if entry not in self.data:
             return False
+        if key is None:
+            return entry in self.data
         if not isinstance(self.data[entry], dict):
             return False
         return key in self.data[entry]
 
 
+class SheerkaDataProviderError(Exception):
+    def __init__(self, message, obj):
+        Exception.__init__(self, message)
+        self.obj = obj
+
+
 class SheerkaDataProvider:
     """Manages the state of the system"""
 
@@ -124,6 +200,7 @@ class SheerkaDataProvider:
     CacheFolder = "cache"
     HeadFile = "HEAD"
     KeysFile = "keys"
+    REF_PREFIX = "##REF##:"
 
     def __init__(self, root=None):
 
@@ -137,37 +214,72 @@ class SheerkaDataProvider:
         self.serializer = Serializer()
 
     def get_obj_path(self, object_type, digest):
-        path.join(self.root, object_type, digest[:24], digest)
+        return path.join(self.root, object_type, digest[:24], digest)
 
-    def add(self, event: Event, entry, obj):
+    @staticmethod
+    def get_obj_key(obj):
+        """
+        Tries to find the key of an object
+        Look for .key, .get_key()
+        :param obj:
+        :return: String version of that is found, None otherwise
+        """
+        return str(obj.key) if hasattr(obj, "key") else str(obj.get_key()) if hasattr(obj, "get_key") else None
+
+    @staticmethod
+    def get_stream_digest(stream):
+        sha256_hash = hashlib.sha256()
+        for byte_block in iter(lambda: stream.read(4096), b""):
+            sha256_hash.update(byte_block)
+
+        stream.seek(0)
+        return sha256_hash.hexdigest()
+
+    @staticmethod
+    def is_reference(obj):
+        return isinstance(obj, str) and obj.startswith(SheerkaDataProvider.REF_PREFIX)
+
+    def add(self, event: Event, entry, obj, allow_multiple=True, use_ref=False):
         """
         Adds obj to the entry 'entry'
         :param event: events that triggers the update of the state
         :param entry: entry of the state to update
         :param obj: obj to insert or add
-        :return: new sha256 of the state
+        :param allow_multiple: if set to true, the same key can be added several times.
+                All entries will be put in a list
+        :param use_ref: if True the actual object is saved under 'objects' folder,
+                only a reference is saved in the state
+        :return: (entry, key) to retrieve the object
         """
+
         event_digest = self.save_event(event)
         snapshot = self.get_snapshot()
         state = self.load_state(snapshot)
 
-        # check uniqueness, cannot add the same key twice
-        obj_key = str(obj.get_key()) if hasattr(obj, "get_key") else None
-        if state.contains(entry, obj_key):
-            raise IndexError(f"{entry}.{obj_key}")
-        elif isinstance(obj, dict):
-            for k in obj:
-                if state.contains(entry, k):
-                    raise IndexError(f"{entry}.{k}")
+        # check uniqueness, cannot add the same key twice if allow_multiple == False
+        key = self.get_obj_key(obj)
+        if not allow_multiple:
+            if isinstance(obj, dict):
+                for k in obj:
+                    if state.contains(entry, k):
+                        raise IndexError(f"{entry}.{k}")
+            else:
+                if state.contains(entry, key):
+                    raise IndexError(f"{entry}.{key}" if key else entry)
 
         state.parents = [] if snapshot is None else [snapshot]
         state.events = [event_digest]
         state.date = datetime.now()
-        state.update(entry, obj if obj_key is None else {obj_key: obj})
+
+        if use_ref:
+            digest = self.save_obj(obj)
+            obj = ObjWithKey(key, self.REF_PREFIX + digest) if key else self.REF_PREFIX + digest
+
+        state.update(entry, obj if (isinstance(obj, ObjWithKey) or key is None) else ObjWithKey(key, obj))
 
         new_snapshot = self.save_state(state)
         self.set_snapshot(new_snapshot)
-        return new_snapshot
+        return entry, key
 
     def add_with_auto_key(self, event: Event, entry, obj):
         """
@@ -180,7 +292,8 @@ class SheerkaDataProvider:
         next_key = self.get_next_key(entry)
         if hasattr(obj, "set_key"):
             obj.set_key(next_key)
-        return self.add(event, entry, {next_key: obj})
+        self.add(event, entry, ObjWithKey(next_key, obj))
+        return entry, next_key
 
     def add_unique(self, event: Event, entry, obj):
         """Add an entry and make sure it's unique"""
@@ -198,14 +311,16 @@ class SheerkaDataProvider:
 
         new_snapshot = self.save_state(state)
         self.set_snapshot(new_snapshot)
-        return new_snapshot
+        return entry, None
 
-    def set(self, event: Event, entry, obj):
+    def set(self, event: Event, entry, obj, use_ref=False):
         """
-        Add or replace an element
+        Add or replace an entry. The entry is reinitialized.
+        If the previous value was dict, all keys are lost
         :param event:
         :param entry:
         :param obj:
+        :param use_ref:
         :return:
         """
         event_digest = self.save_event(event)
@@ -215,22 +330,30 @@ class SheerkaDataProvider:
         state.parents = [] if snapshot is None else [snapshot]
         state.events = [event_digest]
         state.date = datetime.now()
-        obj_key = str(obj.get_key()) if hasattr(obj, "get_key") else None
-        state.update(entry, obj if obj_key is None else {obj_key: obj}, append=False)
+
+        key = self.get_obj_key(obj)
+        obj = self.save_ref_if_needed(use_ref, obj)
+
+        state.data[entry] = obj if key is None else {key: obj}
 
         new_snapshot = self.save_state(state)
         self.set_snapshot(new_snapshot)
-        return new_snapshot
+        return entry, key
 
     def modify(self, event: Event, entry, key, obj):
         """
-        Updates an existing element when element are saved by key
+        Replace an element
+        If the key is not provided, has the same effect than set eg, the entry is reset
         :param event:
         :param entry:
         :param key: key of the object to update
         :param obj: new data
         :return:
         """
+
+        if key is None:
+            raise SheerkaDataProviderError("Key is mandatory.", None)
+
         event_digest = self.save_event(event)
         snapshot = self.get_snapshot()
         state = self.load_state(snapshot)
@@ -244,14 +367,24 @@ class SheerkaDataProvider:
         state.parents = [] if snapshot is None else [snapshot]
         state.events = [event_digest]
         state.date = datetime.now()
-        if key is None:
-            state.data[entry] = obj
+
+        # Gets obj original key, it will help to know if the key has changed
+        obj_key = self.get_obj_key(obj) or key
+
+        if isinstance(state.data[entry][key], list):
+            if not hasattr(obj, Serializer.ORIGIN):
+                raise (SheerkaDataProviderError(f"Multiple entries under '{entry}.{key}'", obj))
+
+            obj_origin = getattr(obj, Serializer.ORIGIN)
+            state.modify_in_list(entry, key, obj, obj_key, obj_origin, self.load_ref_if_needed, self.save_ref_if_needed)
+
         else:
-            state.update(entry, {key: obj})
+            obj = self.save_ref_if_needed(self.is_reference(state.data[entry][key]), obj)
+            state.modify(entry, key, obj, obj_key)
 
         new_snapshot = self.save_state(state)
         self.set_snapshot(new_snapshot)
-        return new_snapshot
+        return entry, obj_key
 
     def list(self, entry, filter=None):
         """
@@ -272,7 +405,7 @@ class SheerkaDataProvider:
             filter_to_use = (lambda k, o: True) if filter is None else filter
             for key, element in elements.items():
                 if filter_to_use(key, element):
-                    yield element
+                    yield self.load_ref_if_needed(element)[0]
         else:
             # manage when no key is defined for the elements
             if not isinstance(elements, list) and not isinstance(elements, set):
@@ -281,7 +414,7 @@ class SheerkaDataProvider:
             filter_to_use = (lambda o: True) if filter is None else filter
             for element in elements:
                 if filter_to_use(element):
-                    yield element
+                    yield self.load_ref_if_needed(element)[0]
 
     def remove(self, event: Event, entry, filter=None):
         """
@@ -325,7 +458,11 @@ class SheerkaDataProvider:
         if key is not None and key not in state.data[entry]:
             raise IndexError(f"{entry}.{key}")
 
-        return state.data[entry] if key is None else state.data[entry][key]
+        item = state.data[entry] if key is None else state.data[entry][key]
+        if isinstance(item, list):
+            return [self.load_ref_if_needed(i)[0] for i in item]
+
+        return self.load_ref_if_needed(item)[0]
 
     def get_safe(self, entry, key=None):
         """
@@ -343,7 +480,7 @@ class SheerkaDataProvider:
         if key is not None and key not in state.data[entry]:
             return None
 
-        return state.data[entry] if key is None else state.data[entry][key]
+        return self.load_ref_if_needed(state.data[entry] if key is None else state.data[entry][key])[0]
 
     def exists(self, entry):
         """
@@ -407,10 +544,9 @@ class SheerkaDataProvider:
             return self.serializer.deserialize(f, None)
 
     def save_obj(self, obj):
-        if hasattr(obj, "key") and hasattr(obj, "key_name") and obj.key is None:
-            obj.key = self.get_next_key(obj.key_name)
+        stream = self.serializer.serialize(obj, SerializerContext(user_name="kodjo"))
+        digest = obj.get_digest() if hasattr(obj, "get_digest") else self.get_stream_digest(stream)
 
-        digest = obj.get_digest()
         target_path = path.join(self.root, SheerkaDataProvider.ObjectsFolder, digest[:24], digest)
         if path.exists(target_path):
             return digest
@@ -419,17 +555,44 @@ class SheerkaDataProvider:
             os.makedirs(path.dirname(target_path))
 
         with open(target_path, "wb") as f:
-            f.write(self.serializer.serialize(obj, SerializerContext("kodjo", digest)).read())
+            f.write(stream.read())
 
         return digest
 
     def load_obj(self, digest):
         if digest is None:
-            return State()
+            return None
 
         target_path = path.join(self.root, SheerkaDataProvider.ObjectsFolder, digest[:24], digest)
+        if not path.exists(target_path):
+            return None
+
         with open(target_path, "rb") as f:
-            return self.serializer.deserialize(f, SerializerContext("kodjo", digest))
+            obj = self.serializer.deserialize(f, SerializerContext(origin=digest))
+
+        # set the origin of the object
+        if not isinstance(obj, str):
+            setattr(obj, Serializer.ORIGIN, digest)
+        return obj
+
+    def load_ref_if_needed(self, obj):
+        if not isinstance(obj, str):
+            return obj, False
+        if not obj.startswith(SheerkaDataProvider.REF_PREFIX):
+            return obj, False
+
+        resolved = self.load_obj(obj[len(SheerkaDataProvider.REF_PREFIX):])
+        if resolved is None:
+            return obj, False
+
+        return resolved, True
+
+    def save_ref_if_needed(self, save_ref, obj):
+        if not save_ref:
+            return obj
+
+        digest = self.save_obj(obj)
+        return self.REF_PREFIX + digest
 
     def get_cache_params(self, category, key):
         digest = hashlib.sha3_256(f"{category}:{key}".encode("utf-8")).hexdigest()
diff --git a/sdp/sheerkaSerializer.py b/sdp/sheerkaSerializer.py
index 8688201..c1ae818 100644
--- a/sdp/sheerkaSerializer.py
+++ b/sdp/sheerkaSerializer.py
@@ -20,8 +20,8 @@ def json_default_converter(o):
 
 @dataclass()
 class SerializerContext:
-    user_name: str
-    origin: str
+    user_name: str = None
+    origin: str = None
 
 
 class Serializer:
@@ -37,7 +37,7 @@ class Serializer:
 
         # add builtin serializers
         self.register(EventSerializer())
-        self.register(PickleSerializer())
+        self.register(StateSerializer())
         self.register(ConceptSerializer())
 
     def register(self, serializer):
@@ -205,11 +205,12 @@ class ObjectSerializer(BaseSerializer):
 
 class PickleSerializer(BaseSerializer):
 
-    def __init__(self):
-        BaseSerializer.__init__(self, "P", 1)
+    def __init__(self, predicate, name="P", version=1):
+        BaseSerializer.__init__(self, name, version)
+        self.predicate = predicate
 
     def match(self, obj):
-        return BaseSerializer.get_full_qualified_name(obj) == "sdp.sheerkaDataProvider.State"
+        return self.predicate(obj)
 
     def dump(self, stream, obj, context):
         stream.write(pickle.dumps(obj))
@@ -220,6 +221,12 @@ class PickleSerializer(BaseSerializer):
         return pickle.loads(stream.read())
 
 
+class StateSerializer(PickleSerializer):
+    def __init__(self, ):
+        PickleSerializer.__init__(self, lambda obj: BaseSerializer.get_full_qualified_name(
+                                      obj) == "sdp.sheerkaDataProvider.State", "S", 1)
+
+
 class ConceptSerializer(ObjectSerializer):
     def __init__(self):
         ObjectSerializer.__init__(self, "core.concept.Concept", "C", 1)
diff --git a/tests/test_sheerkaDataProvider.py b/tests/test_sheerkaDataProvider.py
index 39c0898..02398e5 100644
--- a/tests/test_sheerkaDataProvider.py
+++ b/tests/test_sheerkaDataProvider.py
@@ -3,12 +3,12 @@ import hashlib
 import pytest
 import os
 from os import path
-from sdp.sheerkaDataProvider import SheerkaDataProvider, Event
+from sdp.sheerkaDataProvider import SheerkaDataProvider, Event, SheerkaDataProviderError
 from datetime import date, datetime
 import shutil
 import json
 
-from sdp.sheerkaSerializer import ObjectSerializer, BaseSerializer, Serializer
+from sdp.sheerkaSerializer import ObjectSerializer, BaseSerializer, Serializer, SerializerContext, PickleSerializer
 
 tests_root = path.abspath("../build/tests")
 
@@ -75,7 +75,7 @@ class ObjNoKey:
 
 
 class ObjDumpJson:
-    def __init__(self, key, value):
+    def __init__(self, key=None, value=None):
         self.key = key
         self.value = value
 
@@ -91,6 +91,10 @@ class ObjDumpJson:
         return self.key
 
     def get_digest(self):
+        """
+        Returns the digest of the event
+        :return: hexa form of the sha256
+        """
         return hashlib.sha256(f"Concept:{self.key}{self.value}".encode("utf-8")).hexdigest()
 
     def to_dict(self):
@@ -123,7 +127,7 @@ def test_i_can_init_the_data_provider():
     assert path.exists(path.join(tests_root, ".sheerka"))
 
 
-def test_i_can_add_and_retrieve_an_event():
+def test_i_can_save_and_load_an_event():
     sdp = SheerkaDataProvider(".sheerka")
     event = Event("hello world", date=date(year=2007, month=9, day=10), user="kodjo")
 
@@ -137,17 +141,23 @@ def test_i_can_add_and_retrieve_an_event():
     assert evt.message == "hello world"
 
 
-def test_i_can_add_an_object():
+def test_i_can_add_an_string():
     sdp = SheerkaDataProvider(".sheerka")
     event = Event("cmd add 'foo => bar'")
     event_digest = event.get_digest()
     obj = "foo => bar"
 
-    state_digest = sdp.add(event, "entry", obj)
-    state = sdp.load_state(state_digest)
+    entry, key = sdp.add(event, "entry", obj)
+    last_commit = sdp.get_snapshot()
+    state = sdp.load_state(last_commit)
+    loaded = sdp.get(entry, key)
+
+    assert entry == "entry"
+    assert key is None
+    assert loaded == obj
 
     assert path.exists(path.join(sdp.root, SheerkaDataProvider.EventFolder, event_digest[0:24], event_digest))
-    assert path.exists(path.join(sdp.root, SheerkaDataProvider.StateFolder, state_digest[0:24], state_digest))
+    assert path.exists(path.join(sdp.root, SheerkaDataProvider.StateFolder, last_commit[0:24], last_commit))
     assert path.exists(path.join(sdp.root, SheerkaDataProvider.HeadFile))
 
     assert state.date is not None
@@ -155,121 +165,349 @@ def test_i_can_add_an_object():
     assert state.events == [event_digest]
     assert state.data == {"entry": "foo => bar"}
 
-    assert read_text_file(path.join(sdp.root, SheerkaDataProvider.HeadFile)) == state_digest
+    assert read_text_file(path.join(sdp.root, SheerkaDataProvider.HeadFile)) == last_commit
 
 
-def test_i_can_add_multiple_elements_in_an_entry():
+def test_i_can_add_several_strings_if_allow_multiple_is_true():
+    sdp = SheerkaDataProvider(".sheerka")
+
+    sdp.add(Event("event"), "entry", "foo")
+    sdp.add(Event("event"), "entry", "foo")
+    entry, key = sdp.add(Event("event"), "entry", "bar")
+    loaded = sdp.get(entry, key)
+
+    assert entry == "entry"
+    assert key is None
+    assert loaded == ["foo", "foo", "bar"]
+
+
+def test_i_cannot_add_several_strings_if_allow_multiple_is_false():
+    sdp = SheerkaDataProvider(".sheerka")
+
+    with pytest.raises(IndexError) as index_error:
+        sdp.add(Event("event"), "entry", "foo", False)
+        sdp.add(Event("event"), "entry", "foo", False)
+    assert index_error.value.args[0] == "entry"
+
+
+def test_i_can_add_an_object_with_no_key():
     sdp = SheerkaDataProvider(".sheerka")
     event = Event("cmd add 'foo => bar'")
+    event_digest = event.get_digest()
+    obj = ObjNoKey("a", "b")
 
-    state_digest1 = sdp.add(event, "entry", 1)
-    state1 = sdp.load_state(state_digest1)
+    entry, key = sdp.add(event, "entry", obj)
+    last_commit = sdp.get_snapshot()
+    state = sdp.load_state(last_commit)
+    loaded = sdp.get(entry, key)
 
-    state_digest2 = sdp.add(event, "entry", 2)
-    state2 = sdp.load_state(state_digest2)
+    assert entry == "entry"
+    assert key is None
+    assert loaded == obj
 
-    state_digest3 = sdp.add(event, "entry", 3)
-    state3 = sdp.load_state(state_digest3)
+    assert path.exists(path.join(sdp.root, SheerkaDataProvider.EventFolder, event_digest[0:24], event_digest))
+    assert path.exists(path.join(sdp.root, SheerkaDataProvider.StateFolder, last_commit[0:24], last_commit))
+    assert path.exists(path.join(sdp.root, SheerkaDataProvider.HeadFile))
 
-    assert state1.data == {"entry": 1}
-    assert state2.data == {"entry": [1, 2]}
-    assert state3.data == {"entry": [1, 2, 3]}
+    assert state.date is not None
+    assert state.parents == []
+    assert state.events == [event_digest]
+    assert state.data == {"entry": ObjNoKey("a", "b")}
+
+    assert read_text_file(path.join(sdp.root, SheerkaDataProvider.HeadFile)) == last_commit
 
 
-def test_i_can_add_element_using_auto_generated_key():
+def test_i_can_add_several_obj_no_key_if_allow_multiple_is_true():
+    sdp = SheerkaDataProvider(".sheerka")
+
+    sdp.add(Event("event"), "entry", ObjNoKey("a", "b"))
+    sdp.add(Event("event"), "entry", ObjNoKey("a", "b"))
+    entry, key = sdp.add(Event("event"), "entry", ObjNoKey("c", "d"))
+    loaded = sdp.get(entry, key)
+
+    assert entry == "entry"
+    assert key is None
+    assert loaded == [ObjNoKey("a", "b"), ObjNoKey("a", "b"), ObjNoKey("c", "d")]
+
+
+def test_i_cannot_add_several_obj_no_key_if_allow_multiple_is_false():
+    sdp = SheerkaDataProvider(".sheerka")
+
+    with pytest.raises(IndexError) as index_error:
+        sdp.add(Event("event"), "entry", ObjNoKey("a", "b"), False)
+        sdp.add(Event("event"), "entry", ObjNoKey("c", "d"), False)
+    assert index_error.value.args[0] == "entry"
+
+
+def test_i_can_add_a_dict():
+    sdp = SheerkaDataProvider(".sheerka")
+    event = Event("cmd add 'foo => bar'")
+    event_digest = event.get_digest()
+    obj = {"my_key": "my_value"}
+
+    entry, key = sdp.add(event, "entry", obj)
+    last_commit = sdp.get_snapshot()
+    state = sdp.load_state(last_commit)
+    loaded = sdp.get(entry, key)
+
+    loaded_value = sdp.get(entry, "my_key")  # we can retrieve by key
+
+    assert entry == "entry"
+    assert key is None  # we return None as dict may contains several entries
+    assert loaded == obj
+    assert loaded_value == "my_value"
+
+    assert path.exists(path.join(sdp.root, SheerkaDataProvider.EventFolder, event_digest[0:24], event_digest))
+    assert path.exists(path.join(sdp.root, SheerkaDataProvider.StateFolder, last_commit[0:24], last_commit))
+    assert path.exists(path.join(sdp.root, SheerkaDataProvider.HeadFile))
+
+    assert state.date is not None
+    assert state.parents == []
+    assert state.events == [event_digest]
+    assert state.data == {"entry": obj}
+
+    assert read_text_file(path.join(sdp.root, SheerkaDataProvider.HeadFile)) == last_commit
+
+
+def test_i_can_add_multiple_entries_at_once_with_dict():
+    sdp = SheerkaDataProvider(".sheerka")
+
+    entry, key = sdp.add(Event("event"), "entry", {"my_key1": "value1", "my_key2": "value2"})
+    loaded = sdp.get(entry, key)
+    loaded_value1 = sdp.get(entry, "my_key1")
+    loaded_value2 = sdp.get(entry, "my_key2")
+
+    assert loaded == {"my_key1": "value1", "my_key2": "value2"}
+    assert loaded_value1 == "value1"
+    assert loaded_value2 == "value2"
+
+
+def test_i_can_add_same_key_with_dict_if_allow_multiple_is_true():
+    sdp = SheerkaDataProvider(".sheerka")
+
+    sdp.add(Event("event"), "entry", {"my_key": "my_value"})
+    entry, key = sdp.add(Event("event"), "entry", {"my_key": "my_value"})
+    loaded1 = sdp.get(entry, key)
+
+    entry, key = sdp.add(Event("event"), "entry", {"my_key": "my_value2"})
+    loaded2 = sdp.get(entry, key)
+
+    assert entry == "entry"
+    assert key is None
+    assert loaded1 == {"my_key": ["my_value", "my_value"]}
+    assert loaded2 == {"my_key": ["my_value", "my_value", "my_value2"]}
+
+
+def test_i_cannot_add_same_key_with_dict_if_allow_multiple_is_false():
+    sdp = SheerkaDataProvider(".sheerka")
+
+    with pytest.raises(IndexError) as index_error:
+        sdp.add(Event("event"), "entry", {"my_key": "my_value"}, False)
+        sdp.add(Event("event"), "entry", {"my_key": "my_value2"}, False)
+    assert index_error.value.args[0] == "entry.my_key"
+
+
+def test_i_can_add_object_with_different_key_if_allow_multiple_is_false():
+    sdp = SheerkaDataProvider(".sheerka")
+
+    sdp.add(Event("event"), "entry", {"my_key": "a"}, False)
+    sdp.add(Event("event"), "entry", {"my_key2": "b"}, False)
+
+    assert sdp.get("entry", "my_key") == "a"
+    assert sdp.get("entry", "my_key2") == "b"
+
+
+def test_i_can_add_obj_with_key():
+    sdp = SheerkaDataProvider(".sheerka")
+    event = Event("cmd add 'foo => bar'")
+    event_digest = event.get_digest()
+    obj1 = ObjWithKey("key1", "b")
+    obj2 = ObjSetKey("c", key="key2")
+
+    entry1, key1 = sdp.add(event, "entry", obj1)  # test when key is taken from obj.get_key()
+    entry2, key2 = sdp.add(event, "entry2", obj2)  # test when key is taken from obj.key
+    last_commit = sdp.get_snapshot()
+    state = sdp.load_state(last_commit)
+
+    loaded1 = sdp.get(entry1, key1)
+    loaded2 = sdp.get(entry2, key2)
+
+    assert entry1 == "entry"
+    assert key1 == "key1"
+    assert loaded1 == ObjWithKey("key1", "b")
+    assert entry2 == "entry2"
+    assert key2 == "key2"
+    assert loaded2 == ObjSetKey("c", key="key2")
+
+    assert path.exists(path.join(sdp.root, SheerkaDataProvider.EventFolder, event_digest[0:24], event_digest))
+    assert path.exists(path.join(sdp.root, SheerkaDataProvider.StateFolder, last_commit[0:24], last_commit))
+    assert path.exists(path.join(sdp.root, SheerkaDataProvider.HeadFile))
+
+    assert state.date is not None
+    assert len(state.parents) == 1
+    assert state.events == [event_digest]
+    assert state.data == {"entry": {"key1": obj1}, "entry2": {"key2": obj2}}
+
+    assert read_text_file(path.join(sdp.root, SheerkaDataProvider.HeadFile)) == last_commit
+
+
+def test_i_can_add_objects_with_same_key_if_allow_multiple_is_true():
+    sdp = SheerkaDataProvider(".sheerka")
+
+    sdp.add(Event("event"), "entry", ObjWithKey("my_key", "b"))
+    entry, key = sdp.add(Event("event"), "entry", ObjSetKey("c", key="my_key"))
+    loaded1 = sdp.get(entry, key)
+
+    entry, key = sdp.add(Event("event"), "entry", ObjSetKey("c", key="my_key"))
+    sdp.add(Event("event"), "entry", ObjSetKey("c", key="my_key2"))  # to prove that it does not melt everything
+    loaded2 = sdp.get(entry, key)
+
+    assert entry == "entry"
+    assert key == "my_key"
+    assert loaded1 == [ObjWithKey("my_key", "b"), ObjSetKey("c", key="my_key")]
+    assert loaded2 == [ObjWithKey("my_key", "b"), ObjSetKey("c", key="my_key"), ObjSetKey("c", key="my_key")]
+
+
+def test_i_cannot_add_object_with_same_key_if_allow_multiple_is_false():
+    sdp = SheerkaDataProvider(".sheerka")
+
+    with pytest.raises(IndexError) as index_error:
+        sdp.add(Event("event"), "entry", ObjWithKey("my_key", "b"), False)
+        sdp.add(Event("event"), "entry", ObjSetKey("c", key="my_key"), False)
+    assert index_error.value.args[0] == "entry.my_key"
+
+
+def test_i_can_add_obj_with_key_to_a_list():
+    sdp = SheerkaDataProvider(".sheerka")
+
+    sdp.add(Event("event"), "entry", "foo")
+    sdp.add(Event("event"), "entry", "bar")  # entry is now a list
+    sdp.add(Event("event"), "entry", ObjWithKey("a", "b"))  # this entry must no be taken as a object with a key
+
+    loaded = sdp.get("entry")
+    assert loaded == ["foo", "bar", ObjWithKey("a", "b")]
+
+
+def test_i_cannot_add_obj_with_no_key_when_then_entry_has_keys():
+    sdp = SheerkaDataProvider(".sheerka")
+
+    with pytest.raises(SheerkaDataProviderError) as error:
+        sdp.add(Event("event"), "entry", ObjWithKey("a", "b"))
+        sdp.add(Event("event"), "entry", "foo")
+
+    assert error.value.obj == "foo"
+
+
+def test_i_can_add_string_using_auto_generated_key():
     sdp = SheerkaDataProvider(".sheerka")
     event = Event("cmd add 'foo => bar'")
     key_file = path.join(sdp.root, SheerkaDataProvider.KeysFile)
 
-    sdp.add_with_auto_key(event, "entry1", "foo")
-    sdp.add_with_auto_key(event, "entry1", "bar")
-    sdp.add_with_auto_key(event, "entry2", "baz")
+    entry1, key1 = sdp.add_with_auto_key(event, "entry1", "foo")
+    entry2, key2 = sdp.add_with_auto_key(event, "entry1", "bar")
+    entry3, key3 = sdp.add_with_auto_key(event, "entry2", "baz")
 
     state = sdp.load_state(sdp.get_snapshot())
 
     assert path.exists(key_file)
     assert read_json_file(key_file) == {"entry1": 2, "entry2": 1}
     assert state.data == {"entry1": {"1": "foo", "2": "bar"}, "entry2": {"1": "baz"}}
+    assert entry1 == "entry1"
+    assert entry2 == "entry1"
+    assert entry3 == "entry2"
+    assert key1 == "1"
+    assert key2 == "2"
+    assert key3 == "1"
 
 
-def test_i_can_add_and_auto_set_the_key():
+def test_i_can_add_object_using_auto_generated_key():
     sdp = SheerkaDataProvider(".sheerka")
     event = Event("cmd add 'foo => bar'")
     key_file = path.join(sdp.root, SheerkaDataProvider.KeysFile)
 
-    sdp.add_with_auto_key(event, "entry1", ObjSetKey("foo"))
-    sdp.add_with_auto_key(event, "entry1", ObjSetKey("bar"))
+    entry1, key1 = sdp.add_with_auto_key(event, "entry1", ObjNoKey("a", "b"))
+    entry2, key2 = sdp.add_with_auto_key(event, "entry1", ObjNoKey("a", "b"))
 
     state = sdp.load_state(sdp.get_snapshot())
 
     assert path.exists(key_file)
     assert read_json_file(key_file) == {"entry1": 2}
-    assert state.data == {"entry1": {"1": ObjSetKey("foo", "1"), "2": ObjSetKey("bar", "2")}}
+    assert state.data == {"entry1": {"1": ObjNoKey("a", "b"), "2": ObjNoKey("a", "b")}}
+    assert entry1 == "entry1"
+    assert entry2 == "entry1"
+    assert key1 == "1"
+    assert key2 == "2"
 
 
-def test_i_can_add_an_object_with_its_own_key():
+def test_object_key_is_updated_when_possible_using_auto_generated_key():
     sdp = SheerkaDataProvider(".sheerka")
     event = Event("cmd add 'foo => bar'")
+    key_file = path.join(sdp.root, SheerkaDataProvider.KeysFile)
 
-    sdp.add(event, "entry", ObjWithKey(1, "foo"))
-    sdp.add(event, "entry", ObjWithKey(2, "bar"))
+    entry1, key1 = sdp.add_with_auto_key(event, "entry1", ObjSetKey("foo"))
+    entry2, key2 = sdp.add_with_auto_key(event, "entry1", ObjSetKey("foo"))
 
     state = sdp.load_state(sdp.get_snapshot())
-    assert state.data == {"entry": {"1": ObjWithKey(1, "foo"), "2": ObjWithKey(2, "bar")}}
 
-
-def test_i_can_add_dictionary():
-    sdp = SheerkaDataProvider(".sheerka")
-    event = Event("cmd add 'foo => bar'")
-
-    sdp.add(event, "entry", {"1": "foo"})
-    sdp.add(event, "entry", {"2": "bar"})
-
-    state = sdp.load_state(sdp.get_snapshot())
-    assert state.data == {"entry": {"1": "foo", "2": "bar"}}
-
-
-def test_i_cannot_add_the_same_key_twice():
-    sdp = SheerkaDataProvider(".sheerka")
-    sdp.add(Event("event"), "entry", {"1": "foo"})
-
-    with pytest.raises(IndexError):
-        sdp.add(Event("event"), "entry", {"1": "foo"})
-
-
-def test_i_cannot_add_the_same_element_twice():
-    sdp = SheerkaDataProvider(".sheerka")
-    sdp.add(Event("event"), "entry", ObjWithKey(1, "foo"))
-
-    with pytest.raises(IndexError):
-        sdp.add(Event("event"), "entry", ObjWithKey(1, "foo"))
+    assert path.exists(key_file)
+    assert read_json_file(key_file) == {"entry1": 2}
+    assert state.data == {"entry1": {"1": ObjSetKey("foo", "1"), "2": ObjSetKey("foo", "2")}}
+    assert entry1 == "entry1"
+    assert entry2 == "entry1"
+    assert key1 == "1"
+    assert key2 == "2"
 
 
 def test_i_can_set_objects_with_key():
     sdp = SheerkaDataProvider(".sheerka")
-    sdp.set(Event("event"), "entry", ObjWithKey(1, "foo"))
-    sdp.set(Event("event"), "entry", ObjWithKey(2, "foo"))
+    sdp.add(Event("event"), "entry", ObjWithKey(1, "foo"))
+    entry, key = sdp.set(Event("event"), "entry", ObjWithKey(2, "foo"))
 
     state = sdp.load_state(sdp.get_snapshot())
     assert state.data == {"entry": {"2": ObjWithKey(2, "foo")}}
+    assert entry == "entry"
+    assert key == "2"
 
 
 def test_i_can_set_objects_with_no_key():
     sdp = SheerkaDataProvider(".sheerka")
-    sdp.set(Event("event"), "entry", ObjNoKey(1, "foo"))
-    sdp.set(Event("event"), "entry", ObjNoKey(2, "foo"))
+    sdp.add(Event("event"), "entry", ObjNoKey(1, "foo"))
+    entry, key = sdp.set(Event("event"), "entry", ObjNoKey(2, "foo"))
 
     state = sdp.load_state(sdp.get_snapshot())
     assert state.data == {"entry": ObjNoKey(2, "foo")}
+    assert entry == "entry"
+    assert key is None
 
 
 def test_i_can_set_from_list_to_dict():
     sdp = SheerkaDataProvider(".sheerka")
     sdp.set(Event("event"), "entry", [ObjNoKey(1, "foo"), ObjNoKey(2, "foo")])
-    sdp.set(Event("event"), "entry", {"1": ObjNoKey(1, "foo"), "2": ObjNoKey(2, "foo")})
+    entry, key = sdp.set(Event("event"), "entry", {"1": ObjNoKey(1, "foo"), "2": ObjNoKey(2, "foo")})
 
     state = sdp.load_state(sdp.get_snapshot())
     assert state.data == {"entry": {"1": ObjNoKey(1, "foo"), "2": ObjNoKey(2, "foo")}}
+    assert entry == "entry"
+    assert key is None
+
+
+def test_i_can_set_using_reference():
+    sdp = SheerkaDataProvider(".sheerka")
+    sdp.serializer.register(PickleSerializer(lambda o: isinstance(o, ObjWithKey)))
+    sdp.add(Event("event"), "entry", ObjWithKey(1, "foo"))
+    entry, key = sdp.set(Event("event"), "entry", ObjWithKey(2, "foo"), use_ref=True)
+
+    state = sdp.load_state(sdp.get_snapshot())
+    assert state.data == {"entry": {"2": '##REF##:9b14e03847d73c640f54ea9b46ba62b19e5451ecd300428a225be012ad9f25f9'}}
+    assert entry == "entry"
+    assert key == "2"
+
+    assert path.exists(sdp.get_obj_path(SheerkaDataProvider.ObjectsFolder,
+                                        "9b14e03847d73c640f54ea9b46ba62b19e5451ecd300428a225be012ad9f25f9"))
+
+    # sanity check, make sure that I can load back
+    loaded = sdp.get(entry, key)
+    assert loaded == ObjWithKey(2, "foo")
 
 
 def test_i_can_add_unique():
@@ -277,10 +515,28 @@ def test_i_can_add_unique():
     sdp.add_unique(Event("event"), "entry", ObjNoKey(1, "foo"))
     sdp.add_unique(Event("event"), "entry", ObjNoKey(1, "foo"))
     sdp.add_unique(Event("event"), "entry", ObjNoKey(2, "bar"))
-    sdp.add_unique(Event("event"), "entry", ObjNoKey(2, "bar"))
+    entry, key = sdp.add_unique(Event("event"), "entry", ObjNoKey(2, "bar"))
 
     state = sdp.load_state(sdp.get_snapshot())
     assert state.data == {"entry": {ObjNoKey(1, "foo"), ObjNoKey(2, "bar")}}
+    assert entry == "entry"
+    assert key is None
+
+
+def test_i_can_add_reference_of_an_object_with_a_key():
+    sdp = SheerkaDataProvider(".sheerka")
+    obj = ObjDumpJson("my_key", "value1")
+    obj_serializer = ObjectSerializer(BaseSerializer.get_full_qualified_name(obj))
+    sdp.serializer.register(obj_serializer)
+
+    entry, key = sdp.add(Event("event"), "entry", obj, use_ref=True)
+    state = sdp.load_state(sdp.get_snapshot())
+    digest = state.data["entry"]["my_key"][len(SheerkaDataProvider.REF_PREFIX):]
+
+    assert key == obj.key
+    assert entry == "entry"
+    assert state.data == {'entry': {'my_key': f"{SheerkaDataProvider.REF_PREFIX}{digest}"}}
+    assert sdp.load_obj(digest) == obj
 
 
 def test_i_can_keep_state_history():
@@ -289,12 +545,14 @@ def test_i_can_keep_state_history():
     event1 = Event("cmd add 'foo => bar'")
     event_digest1 = event1.get_digest()
     obj1 = "foo => bar"
-    state_digest1 = sdp.add(event1, "entry1", obj1)
+    sdp.add(event1, "entry1", obj1)
+    state_digest1 = sdp.get_snapshot()
 
     event2 = Event("cmd add 'foo => baz'")
     event_digest2 = event2.get_digest()
     obj2 = "foo => baz"
-    state_digest2 = sdp.add(event2, "entry2", obj2)
+    sdp.add(event2, "entry2", obj2)
+    state_digest2 = sdp.get_snapshot()
 
     state2 = sdp.load_state(state_digest2)
 
@@ -318,24 +576,30 @@ def test_i_can_list_elements_when_there_is_nothing_to_list():
 
 def test_i_can_list_when_no_key():
     sdp = SheerkaDataProvider(".sheerka")
+    sdp.serializer.register(PickleSerializer(lambda obj: isinstance(obj, str)))
+
     sdp.add(Event("event"), "entry1", "foo")
     sdp.add(Event("event"), "entry1", "bar")
-    sdp.add(Event("event"), "entry2", "baz")
+    sdp.add(Event("event"), "entry1", "baz", use_ref=True)
+    sdp.add(Event("event"), "entry2", "xyz")
 
     result = sdp.list("entry1")
 
-    assert list(result) == ["foo", "bar"]
+    assert list(result) == ["foo", "bar", "baz"]
 
 
 def test_i_can_list_when_key():
     sdp = SheerkaDataProvider(".sheerka")
+    sdp.serializer.register(PickleSerializer(lambda obj: isinstance(obj, ObjWithKey)))
+
     sdp.add(Event("event"), "entry1", {"1": "foo"})
     sdp.add(Event("event"), "entry1", {"2": "bar"})
-    sdp.add(Event("event"), "entry2", {"3": "baz"})
+    sdp.add(Event("event"), "entry1", ObjWithKey("3", "value"), use_ref=True)
+    sdp.add(Event("event"), "entry2", {"4": "xxx"})
 
     result = sdp.list("entry1")
 
-    assert list(result) == ["foo", "bar"]
+    assert list(result) == ["foo", "bar", ObjWithKey("3", "value")]
 
 
 def test_i_can_list_when_one_element():
@@ -459,32 +723,234 @@ def test_i_cannot_remove_if_entry_does_not_exist():
         assert str(e) == "entry"
 
 
-def test_i_can_replace_an_entry():
+def test_i_cannot_modify_an_entry_without_a_key():
     sdp = SheerkaDataProvider(".sheerka")
-    sdp.add(Event("event"), "entry1", "foo")
-    sdp.add(Event("event"), "entry1", "bar")
 
-    sdp.modify(Event("event"), "entry1", None, "baz")
-    result = sdp.list("entry1")
+    with pytest.raises(SheerkaDataProviderError) as error:
+        sdp.modify(Event("event"), "entry", None, "baz")
 
-    assert list(result) == ["baz"]
+    assert error.value.args[0] == "Key is mandatory."
 
 
-def test_i_cannot_update_an_entry_that_does_not_exist():
+def test_i_can_modify_dict_with_a_key():
+    sdp = SheerkaDataProvider(".sheerka")
+    sdp.add(Event("event"), "entry", {"key1": "foo"})
+    sdp.add(Event("event"), "entry", {"key2": "bar"})
+
+    entry, key = sdp.modify(Event("event"), "entry", "key1", "baz")
+
+    state = sdp.load_state(sdp.get_snapshot())
+    assert state.data == {"entry": {"key1": "baz", "key2": "bar"}}
+    assert entry == "entry"
+    assert key == "key1"
+
+
+def test_i_can_modify_an_object_with_a_key():
+    sdp = SheerkaDataProvider(".sheerka")
+    sdp.add(Event("event"), "entry", ObjWithKey("key1", "foo"))
+    sdp.add(Event("event"), "entry", ObjWithKey("key2", "bar"))
+
+    entry, key = sdp.modify(Event("event"), "entry", "key1", ObjWithKey("key1", "baz"))
+
+    state = sdp.load_state(sdp.get_snapshot())
+    assert state.data == {"entry": {"key1": ObjWithKey("key1", "baz"), "key2": ObjWithKey("key2", "bar")}}
+    assert entry == "entry"
+    assert key == "key1"
+
+
+def test_i_can_modify_an_object_while_changing_the_key():
+    sdp = SheerkaDataProvider(".sheerka")
+    sdp.add(Event("event"), "entry", ObjWithKey("key1", "foo"))
+    sdp.add(Event("event"), "entry", ObjWithKey("key2", "bar"))
+
+    entry, key = sdp.modify(Event("event"), "entry", "key1", ObjWithKey("key3", "baz"))
+
+    state = sdp.load_state(sdp.get_snapshot())
+    assert state.data == {"entry": {"key2": ObjWithKey("key2", "bar"), "key3": ObjWithKey("key3", "baz")}}
+    assert entry == "entry"
+    assert key == "key3"
+
+
+def test_i_can_modify_an_object_while_changing_the_key_to_an_existing_key():
+    sdp = SheerkaDataProvider(".sheerka")
+    sdp.add(Event("event"), "entry", ObjWithKey("key1", "foo"))
+    sdp.add(Event("event"), "entry", ObjWithKey("key2", "bar"))
+
+    entry, key = sdp.modify(Event("event"), "entry", "key2", ObjWithKey("key1", "bar"))
+
+    state = sdp.load_state(sdp.get_snapshot())
+    assert state.data == {"entry": {"key1": [ObjWithKey("key1", "foo"), ObjWithKey("key1", "bar")]}}
+    assert entry == "entry"
+    assert key == "key1"
+
+
+def test_i_can_modify_an_object_while_changing_the_key_to_an_existing_when_list():
+    """
+    In this example, the item to modify is within a list, and its key has changed
+    and in the new key, there is already a list
+    :return:
+    """
+    sdp = SheerkaDataProvider(".sheerka")
+    sdp.serializer.register(PickleSerializer(lambda o: isinstance(o, ObjDumpJson)))
+
+    sdp.add(Event("event"), "entry", ObjDumpJson("key1", "value11"))
+    sdp.add(Event("event"), "entry", ObjDumpJson("key1", "value12"))
+    sdp.add(Event("event"), "entry", ObjDumpJson("key2", "value21"))
+    sdp.add(Event("event"), "entry", ObjDumpJson("key2", "value22"))
+
+    new_value = ObjDumpJson("key1", "value13")
+    setattr(new_value, Serializer.ORIGIN, ObjDumpJson("key2", "value21").get_digest())
+    entry, key = sdp.modify(Event("event"), "entry", "key2", new_value)
+
+    state = sdp.load_state(sdp.get_snapshot())
+    assert state.data == {"entry": {
+        "key1": [ObjDumpJson("key1", "value11"), ObjDumpJson("key1", "value12"), ObjDumpJson("key1", "value13")],
+        "key2": [ObjDumpJson("key2", "value22")]
+    }}
+    assert entry == "entry"
+    assert key == "key1"
+
+
+def test_i_can_modify_an_object_while_changing_the_key_to_an_existing_when_nothing():
+    """
+    In this example, the item to modify is within a list, and its key has changed
+    and in the new key, there is nothing (the new key does not exist)
+    :return:
+    """
+    sdp = SheerkaDataProvider(".sheerka")
+    sdp.serializer.register(PickleSerializer(lambda o: isinstance(o, ObjDumpJson)))
+
+    sdp.add(Event("event"), "entry", ObjDumpJson("key2", "value21"))
+    sdp.add(Event("event"), "entry", ObjDumpJson("key2", "value22"))
+
+    new_value = ObjDumpJson("key1", "value13")
+    setattr(new_value, Serializer.ORIGIN, ObjDumpJson("key2", "value21").get_digest())
+    entry, key = sdp.modify(Event("event"), "entry", "key2", new_value)
+
+    state = sdp.load_state(sdp.get_snapshot())
+    assert state.data == {"entry": {
+        "key1": ObjDumpJson("key1", "value13"),
+        "key2": [ObjDumpJson("key2", "value22")]
+    }}
+    assert entry == "entry"
+    assert key == "key1"
+
+
+def test_i_can_modify_an_object_while_changing_the_key_to_an_existing_when_one_item():
+    """
+    In this example, the item to modify is within a list, and its key has changed
+    and in the new key, there is only one element
+    :return:
+    """
+    sdp = SheerkaDataProvider(".sheerka")
+    sdp.serializer.register(PickleSerializer(lambda o: isinstance(o, ObjDumpJson)))
+
+    sdp.add(Event("event"), "entry", ObjDumpJson("key1", "value11"))
+    sdp.add(Event("event"), "entry", ObjDumpJson("key2", "value21"))
+    sdp.add(Event("event"), "entry", ObjDumpJson("key2", "value22"))
+
+    new_value = ObjDumpJson("key1", "value13")
+    setattr(new_value, Serializer.ORIGIN, ObjDumpJson("key2", "value21").get_digest())
+    entry, key = sdp.modify(Event("event"), "entry", "key2", new_value)
+
+    state = sdp.load_state(sdp.get_snapshot())
+    assert state.data == {"entry": {
+        "key1": [ObjDumpJson("key1", "value11"), ObjDumpJson("key1", "value13")],
+        "key2": [ObjDumpJson("key2", "value22")]
+    }}
+    assert entry == "entry"
+    assert key == "key1"
+
+
+def test_i_can_modify_a_ref():
+    sdp = SheerkaDataProvider(".sheerka")
+    sdp.serializer.register(PickleSerializer(lambda o: isinstance(o, ObjWithKey)))
+    sdp.add(Event("event"), "entry", ObjWithKey("key1", "foo"))
+    entry, key = sdp.add(Event("event"), "entry", ObjWithKey("key2", "bar"), use_ref=True)
+
+    sdp.modify(Event("event"), "entry", "key2", ObjWithKey("key2", "baz"))
+    state = sdp.load_state(sdp.get_snapshot())
+    assert state.data == {"entry": {
+        "key1": ObjWithKey("key1", "foo"),
+        "key2": "##REF##:eb297e98710dd17244bb0e38eb9f1bf72cba692a8f8d94e9eb2d898e130cac8b"}}
+    assert entry == "entry"
+    assert key == "key2"
+
+
+def test_i_cannot_modify_an_entry_that_does_not_exist():
     sdp = SheerkaDataProvider(".sheerka")
 
     with pytest.raises(IndexError) as e:
         sdp.modify(Event("event"), "entry", "key", "foo")
-        assert str(e) == "entry"
+
+    assert str(e.value) == "entry"
 
 
-def test_i_cannot_update_a_key_that_does_not_exist():
+def test_i_cannot_modify_a_key_that_does_not_exist():
     sdp = SheerkaDataProvider(".sheerka")
     sdp.add(Event("event"), "entry1", {"1": "foo"})
 
     with pytest.raises(IndexError) as e:
         sdp.modify(Event("event"), "entry1", "2", "bar")
-        assert str(e) == "entry.1"
+        assert str(e) == "entry1.2"
+
+
+def test_i_cannot_modify_a_list_when_origin_is_unknown():
+    sdp = SheerkaDataProvider(".sheerka")
+
+    sdp.add(Event("event"), "entry", ObjWithKey("key", "value1"))
+    sdp.add(Event("event"), "entry", ObjWithKey("key", "value2"))  # same they
+
+    state = sdp.load_state(sdp.get_snapshot())
+
+    with pytest.raises(SheerkaDataProviderError) as error:
+        sdp.modify(Event("event"), "entry", "key", ObjWithKey("key", "value2"))
+
+    assert error.value.obj == ObjWithKey("key", "value2")
+    assert error.value.args[0] == "Multiple entries under 'entry.key'"
+
+
+def test_i_can_modify_a_list_when_the_origin_is_known():
+    sdp = SheerkaDataProvider(".sheerka")
+
+    sdp.add(Event("event"), "entry", ObjDumpJson("key", "value1"))
+    sdp.add(Event("event"), "entry", ObjDumpJson("key", "value2"))  # same they
+
+    new_value = ObjDumpJson("key", "value3")
+    setattr(new_value, Serializer.ORIGIN, ObjDumpJson("key", "value1").get_digest())
+
+    sdp.modify(Event("event"), "entry", "key", new_value)
+
+    state = sdp.load_state(sdp.get_snapshot())
+    assert state.data == {"entry": {"key": [ObjDumpJson("key", "value3"), ObjDumpJson("key", "value2")]}}
+
+
+def test_i_can_modify_a_list_when_the_origin_is_known_2():
+    """
+    This time, we check that the origin is automatically set when the object was saved as a reference
+    We also check that all objects are still persisted as reference
+    :return:
+    """
+    sdp = SheerkaDataProvider(".sheerka")
+    sdp.serializer.register(PickleSerializer(lambda o: isinstance(o, ObjDumpJson)))
+
+    sdp.add(Event("event"), "entry", ObjDumpJson("key", "value1"), use_ref=True)
+    sdp.add(Event("event"), "entry", ObjDumpJson("key", "value2"), use_ref=True)  # same they
+
+    objs = sdp.get("entry", "key")  # origin is automatically set to the loaded objects
+    objs[0].value = "value3"
+
+    sdp.modify(Event("event"), "entry", "key", objs[0])
+
+    state = sdp.load_state(sdp.get_snapshot())
+    assert state.data == {"entry": {"key": [
+        "##REF##:621771a3af6a331e9abb3a63fb25e0cac4b13df0b292dfa30db6bd89031bfad0",
+        "##REF##:5fe085e8366d35c5f04a18b2d3dada376128b246e07c66de5872830b00f5f517"]}}
+
+    # checks that all objects are (still) persisted
+    path.exists(sdp.get_obj_path(sdp.ObjectsFolder, "621771a3af6a331e9abb3a63fb25e0cac4b13df0b292dfa30db6bd89031bfad0"))
+    path.exists(sdp.get_obj_path(sdp.ObjectsFolder, "5fe085e8366d35c5f04a18b2d3dada376128b246e07c66de5872830b00f5f517"))
+    path.exists(sdp.get_obj_path(sdp.ObjectsFolder, "1aac9e0d5c74c3bb989fd0f9def792bba36c5595d32f61be7cbb1a38dcf75327"))
 
 
 def test_i_can_get_the_entire_entry():
@@ -522,6 +988,30 @@ def test_i_can_get_an_entry_by_key():
     assert result_safe == "bar"
 
 
+def test_i_can_get_object_save_by_reference():
+    sdp = SheerkaDataProvider(".sheerka")
+    obj = ObjDumpJson("my_key", "value1")
+    sdp.serializer.register(ObjectSerializer(BaseSerializer.get_full_qualified_name(obj)))
+
+    entry, key = sdp.add(Event("event"), "entry", obj, use_ref=True)
+    loaded = sdp.get(entry, key)
+
+    assert loaded == obj
+
+
+def test_i_can_get_objects_from_list_when_saved_by_reference():
+    sdp = SheerkaDataProvider(".sheerka")
+    sdp.serializer.register(PickleSerializer(lambda o: isinstance(o, ObjDumpJson)))
+
+    sdp.add(Event("event"), "entry", ObjDumpJson("key", "value1"), use_ref=True)
+    sdp.add(Event("event"), "entry", ObjDumpJson("key", "value2"), use_ref=True)  # same they
+
+    objs = sdp.get("entry", "key")
+
+    assert objs[0] == ObjDumpJson("key", "value1")
+    assert objs[1] == ObjDumpJson("key", "value2")
+
+
 def test_i_cannot_get_an_entry_that_does_not_exist():
     sdp = SheerkaDataProvider(".sheerka")
 
@@ -604,53 +1094,52 @@ def test_i_can_test_than_an_entry_exits():
     assert sdp.exists("entry")
 
 
-def test_i_can_save_and_load_object_with_history():
+def test_i_can_save_and_load_object_ref_with_history():
     sdp = SheerkaDataProvider(".sheerka")
     obj = ObjDumpJson("my_key", "value1")
     sdp.serializer.register(ObjectSerializer(BaseSerializer.get_full_qualified_name(obj)))
 
-    entry, key = sdp.add_ref("Obj", obj)
+    entry, key = sdp.add(Event("event"), "entry", obj, use_ref=True)
     loaded = sdp.get(entry, key)
     history = getattr(loaded, Serializer.HISTORY)
 
     assert key == obj.key
-    assert entry == "Obj"
+    assert entry == "entry"
     assert loaded.key == obj.key
     assert loaded.value == obj.value
 
-    assert getattr(history, Serializer.USERNAME) == "kodjo"
-    assert getattr(history, Serializer.MODIFICATION_DATE) != ""
-    assert getattr(history, Serializer.PARENTS) == []
+    assert history[Serializer.USERNAME] == "kodjo"
+    assert history[Serializer.MODIFICATION_DATE] != ""
+    assert history[Serializer.PARENTS] == []
 
     assert os.path.exists(sdp.get_obj_path(sdp.ObjectsFolder, obj.get_digest()))
 
     # save a second type with no modification
-    previous_modification_time = getattr(history, Serializer.MODIFICATION_DATE)
-    previous_parents = getattr(history, Serializer.PARENTS)
+    previous_modification_time = history[Serializer.MODIFICATION_DATE]
+    previous_parents = history[Serializer.PARENTS]
 
-    sdp.add_ref("Obj", loaded)
+    sdp.modify(Event("event"), "entry", key, loaded)
     loaded = sdp.get(entry, key)
     history = getattr(loaded, Serializer.HISTORY)
 
-    assert getattr(history, Serializer.MODIFICATION_DATE) == previous_modification_time
-    assert getattr(history, Serializer.PARENTS) == previous_parents
+    assert history[Serializer.MODIFICATION_DATE] == previous_modification_time
+    assert history[Serializer.PARENTS] == previous_parents
 
     # save again, but with a modification
     previous_digest = loaded.get_digest()
     loaded.value = "value2"
 
-    sdp.add_ref("Obj", loaded)
+    sdp.modify(Event("event"), "entry", key, loaded)
     loaded2 = sdp.get(entry, key)
-    history2 = getattr(loaded, Serializer.HISTORY)
+    history2 = getattr(loaded2, Serializer.HISTORY)
 
     assert loaded2.key == loaded.key
     assert loaded2.value == loaded.value
 
-    assert getattr(history2, Serializer.USERNAME) == "kodjo"
-    assert getattr(history2, Serializer.MODIFICATION_DATE) != ""
-    assert getattr(history2, Serializer.PARENTS) == [previous_digest]
-
-
-
-
+    assert history2[Serializer.USERNAME] == "kodjo"
+    assert history2[Serializer.MODIFICATION_DATE] != ""
+    assert history2[Serializer.PARENTS] == [previous_digest]
 
+    state = sdp.load_state(sdp.get_snapshot())
+    assert state.data == {"entry": {
+        "my_key": '##REF##:e6bf5b56428cfce0f08c94f2c3625dc3b3a8180d7229eaa9f8aa967fb16e5256'}}