Support for DictField of ReferenceField with compound primary key

Hello,

There seems to be a problem when trying to use a Dictfield of ReferenceFields with a compound key. I have the following snippet:

class Key(me.EmbeddedDocument):
    name = me.StringField()
    revision = me.IntField()

class B(me.Document):
    key = me.EmbeddedDocumentField(Key, primary_key=True)
    comment = me.StringField()

class A(me.Document):
    description = me.StringField()
    b = me.DictField(me.ReferenceField(B, dbref=True))

if __name__ == "__main__":
    db = me.connect('Test', host="mongodb://localhost/test")

    a = A(description="description")

    b = B(comment="comment", key={"name": "name", "revision": 0})
    b.save()

    a.b["name"] = b

    a.save()

    b = B.objects().first()
    print(b.key.revision)

    a = A.objects().first()

    print(a.b["name"].key.revision)

Here is the output I get:

0
Traceback (most recent call last):
  File ".\debug_instrument.py", line 69, in <module>
    print('a.b["name"].key.revision', a.b["name"].key.revision)
  File "C:\Users\ezalczer\AppData\Local\Programs\Python\Python37\lib\site-packages\mongoengine\base\fields.py", line 340, in __get__
    value = _dereference(value, max_depth=1, instance=instance, name=self.name)
  File "C:\Users\ezalczer\AppData\Local\Programs\Python\Python37\lib\site-packages\mongoengine\dereference.py", line 98, in __call__
    self.reference_map = self._find_references(items)
  File "C:\Users\ezalczer\AppData\Local\Programs\Python\Python37\lib\site-packages\mongoengine\dereference.py", line 153, in _find_references
    reference_map.setdefault(item.collection, set()).add(item.id)
TypeError: unhashable type: 'dict'

This issue happens in dereference.py and is caused by the fact that we try to add item.id to a set in reference_map. In this case, item.id is a dictionary, which is not immutable. I'm not sure how to go about that, there should be a way to work around it (for instance convert dict.items to a tuple) but I would like your opinion before I try to implement anything. Or perhaps I am missing something and there is already a way to do that ?

Thanks

Start in dereference.py, particularly _find_references, and reproduce the supplied DictField/ReferenceField example with the compound primary key. Trace how item.id is collected in reference_map and verify that dereferencing the stored reference no longer raises the reported TypeError. Done means the example can read a.b["name"].key.revision successfully.