Provide hook for bidirectional relationship for embedded items?

Hi everyone, great work on MongoEngine.

Here's something I have yet to find but really want. The ability to have a function called after the object is created and restored from MongoDB but not yet returned from the query. I think this would be super useful.

Something like:

class Chapter(Document):
    # standard stuff here

   def _loaded(self):
        # this function is called in the situation described 
        # above as part of deserialization
        pass

Here's my use-case. I have two classes:

class Chapter(Document):
    # other fields...
    sections = ListField(EmbeddedDocumentField(Section))

class Section(EmbeddedDocument):
    chapter = None
    # other fields...

I want to associate the section back with it's owning chapter link this:

class Chapter(Document):
    # other fields...
    sections = ListField(EmbeddedDocumentField(Section))

      def _loaded(self):
          for sec in self.sections:
                sec.chapter = self

That way given a chapter I can navigate the sections (already) and given a section, I can get back to the chapter without touching the DB.

Start by tracing MongoEngine's deserialization path for Document and EmbeddedDocument objects. Define and test the lifecycle point where the callback runs after restoration but before the query returns, including the Chapter-to-Section back-reference use case.