Simulated ForeignKeys from MongoEngine Document to Django Models

gistfile1.py

# Using the `@property` decorator allows the MongoEngine Document object to have
# properties that reference Models, and be got/set like normal foreign keys.
# This same pattern can work the other way and allow Models interface with
# Documents.

class Foo(mongoengine.Document):
    # The `id` of the property is stored in the Document.
    user_id         = mongoengine.IntField()

    # Setters and getters to interface with the SQL DB and set Users to the
    # user property directly.
    @property
    def user(self):
        # Lazily dereference the Model, and cache the object in the `_data`
        # property to avoid repeated fetching.
        if 'user' not in self._data:
            self._data['user'] = User.objects.get(pk=self.user_id)
        return self._data['user']
    
    @user.setter
    def user(self, value):
        # Cache the value in the `_data` attribute and set the corresponding
        # `id` property.
        if value and hasattr(value, 'pk'):
            self._data['user'] = value
            self.user_id = value.pk
        else:
            self._data['user'] = None
            self.user_id = None
    
    @user.deleter
    def user(self):
        self._data['user'] = None
        self.user_id = None

Yeah I don't blame you: I'm using PyMongo directly for some things (particularly where speed is a concern). Anyway I think it's even simpler than that, you can basically just write a custom property decorator that sets the attribute on the given model in the scope of the fget method. So long as this is executed at some point, because it's using the customized mongoengine descriptor logic, it will actually be able to query by it. One issue with this approach is that it doesn't work until it's been initialized via getattr for example. Anyway thanks for indulging me.