Possibility of embedding a clone of a document as embedded

[aop]

My current issue arises from the need of snapshotting documents for later use.

Say I have a

    class User(Document):
        name = StringField()

and a

    class Message(Document):
        message = StringField()
        user = DynamicField()

For every message that the user sends I'd like to save a copy of the user in case the user changes his name, I'd see what the user looked like when he sent the message.
The example might not make any sense but the idea should be clear. Previously (I think < 0.10) I could create a Document by inheriting both Document and EmbeddedDocument, but for some reason that is not possible any more.

How could I implement snapshotting of Documents as EmbeddedDocuments, or if not elegantly possible, it would be a nice feature. I think making multiple inheritance from Document and EmbeddedDocument would be best.

[touilleMan]

Hi,

First I've updated your comment to use markdown code blocks for with python syntax.

I tried to do something similar (implementing an history system) using the to_mongo method to create a dump of the document:

class User(Document):
    name = StringField()

class Message(Document):
    message = StringField()
    user = DictField()

# User provide a new message
doc = Doc.objects.get(id=id)
# doc.to_mongo().to_dict() serializes doc into a dict
msg = Message(message="Hi there !", user=doc.to_mongo().to_dict())
msg.save()

However, I bumped into a bug (see #1058), basically the user DictField is really similar to a ReferenceField and mongoengine try to dereference it... then weird things happened !

So for now I switched to a less elegant solution:

...
class Message(Document):
    message = StringField()
    user = StringField()

# User provides a new message
doc = Doc.objects.get(id=id)
# to_json returns a string containing the json encoded dump of the document
msg = Message(message="Hi there !", user=doc.to_json())
msg.save()

Would it be possible to revert to previous behavior and allow both inheriting from Document and EmbeddedDocument? It was the feature I'd appreciate most from MongoEngine. It is really useful to embed shapshot in objects while keeping the code clean

[AlexanderPease]

+1 this feels like a super native feature of MongoDB that MongoEngine should support

[brianleipprandt]

Would a CachedReferenceField with auto_sync=False work for this?

http://docs.mongoengine.org/apireference.html#mongoengine.fields.CachedReferenceField
Original discussion for CachedReferenceField: #698

[wojcikstefan]

I think making multiple inheritance from Document and EmbeddedDocument would be best.

I touched on this point in #1006 (comment). TL;DR I don't think it's a good/sustainable idea.

However, it would be nice to have an easy way to transfer the data from a document to an embedded doc. Right now, something like this works:

In [2]: from mongoengine import *

In [3]: connect('testdb')
Out[3]: MongoClient(host=['localhost:27017'], document_class=dict, tz_aware=False, connect=True, read_preference=Primary())

In [4]: class BaseAddress(object):
   ...:     street_address = StringField()
   ...:     city = StringField()
   ...:     state_province = StringField()
   ...:     country = StringField()
   ...:

In [5]: class Address(BaseAddress, Document):
   ...:     pass
   ...:

In [6]: class Address(BaseAddress, Document):
   ...:     pass
   ...:

In [7]: class EmbeddedAddress(BaseAddress, EmbeddedDocument):
   ...:     pass
   ...:

In [8]: class User(Document):
   ...:     embedded_address = EmbeddedDocumentField(EmbeddedAddress)
   ...:     ref_address = ReferenceField(Address)
   ...:

In [9]: address = Address.objects.create(city='San Francisco', street_address='1355 Market St')

In [10]: {field_name: address[field_name] for field_name in address}
Out[10]:
{'city': u'San Francisco',
 'country': None,
 'id': ObjectId('5863587dc6e47b22b271f13b'),
 'state_province': None,
 'street_address': u'1355 Market St'}

In [11]: embedded = EmbeddedAddress(**{field_name: address[field_name] for field_name in address})

In [12]: embedded._data
Out[12]: {"street_address": u'1355 Market St', "city": u'San Francisco', "state_province": None, "country": None}

In [13]: User.objects.create(embedded_address=embedded, ref_address=address)
Out[13]: <User: User object>