In mongoengine what value must be set into ReferenceField. I mean should we provide in "ObjectId" of the document to which the reference is to be made. For example,
class Bar(Document):
content = StringField()
foo = ReferenceField('Foo')
The object of class Bar should have what value set to in "foo" attribute. Should it be the ObjectId of some document in 'Foo' collection? Also can I set any other unique field as a value in reference field mentioning which field it is?