[source] implement storages right in the source rather than in hooks The problem is that Storage objects will most probably change entity's dictionary so that values are correctly set before the source's corresponding method (e.g. entity_added()) is called. For instance, the BFSFileStorage will change the original binary data and replace it with the destination file path in order to store the file path in the database. This change must be local to the source in order not to impact other hooks or attribute access during the transaction, the whole idea being that the same application code should work exactly the same whether or not a BFSStorage is used or not.