-
-
Save alecperkins/1364430 to your computer and use it in GitHub Desktop.
# 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 |
Sure but what I'm getting at is using a custom descriptor to actually set the fields dynamically. That would be a huge boon to situations where you might want to do something complex and dynamic that should be query-able. It's unfortunate that mongoengine doesn't already provide a facility like this but I imagine that it could be done by hooking into the mechanics of setting a field.
You can. Just make your own derivative of BaseField
and define the necessary to_mongo
and to_python
methods that do the translation, as specific to your data model. This simulated foreign key thing could be done as a custom field, but it was simpler to use the @property
decorator. (I've actually since moved away from using MongoEngine entirely — just using PyMongo directly, with DictShield for validation help.)
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.
The non-field attributes don't exist at the database level, though. In Mongo, the above document looks something like:
Mongoengine operates at the application level, mapping that document to a Foo object, and vice versa, using the
_types
and_cls
values to know what object class to use. Theuser
property is merely a convenience in the application level that avoids having to duplicate theUser.objects.get(pk=obj.user_id)
lookup all over the place.