Add documentation about database versioning

docs/development/design.rst

@@ -162,6 +162,31 @@ has the proper access (based on the network)::
         if not current_user.has_access_to_network(interface.network):
             abort(403)
 
+Database versioning
+-------------------
+
+SQLAlchemy-Continuum_ is used to track changes and keep an history.
+
+To enable versioning on a models, the ``__versioned__`` attribute shall be added to the model class.
+The following classes are versioned:
+
+-  :class:`~app.models.Item`
+-  :class:`~app.models.Host`
+-  :class:`~app.models.AnsibleGroup`
+
+The *History* on the view host or view group page displays the list of changes performed.
+As there is a relationship between Host and AnsibleGroup, it can lead to unexpected behavior.
+
+1. Edit a group to add a host -> new host added to the group history
+2. Edit another host to add it to that group -> new group added to the host history
+
+At this point the group page will show 2 hosts but only one was added in the history.
+If the group is edited, to add a variable, the recorded change will display all current hosts in the history.
+This is correct as those hosts were present when the group was edited.
+But the information showing the hosts were added previously is missing.
+This is a limitation of the current implementation.
+
+
 
 .. _Flask: https://flask.palletsprojects.com
 .. _The Flask Mega-Tutorial: https://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-part-i-hello-world
@@ -171,3 +196,4 @@ has the proper access (based on the network)::
 .. _Elasticsearch: https://www.elastic.co/elasticsearch/
 .. _RQ: https://python-rq.org
 .. _alembic: https://alembic.sqlalchemy.org
+.. _SQLAlchemy-Continuum: https://sqlalchemy-continuum.readthedocs.io/en/latest/