Following backreferences of unknown kinds in NDB

Question

I'm in the process of writing my first RESTful web service atop GAE and the Python 2.7 runtime; I've started out using Guido's shiny new ndb API. However, I'm unsure how to solve a particular case without the implicit back-reference feature of the original db API. If the user-agent requests a particular resource and those resources 1 degree removed: host/api/kind/id?depth=2 What's the best way to discover a related collection of entities from the "one" in a one-to-many relationship, given that the kind of the related entity is unknown at development time? I'm unable to use a replacement query as described in a previous SO inquiry due to the latter restriction. The fact that my model is definable at runtime (and therefore isn't hardcoded) prevents me from using a query to filter properties for matching keys. Ancestor and other kindless queries are also out due to the datastore limitation that prevents me from filtering on a property without the kind specified. Thus far, the only idea I…

Accepted Answer

Interesting question! So basically you want to look at the Contact class and find out if there is some other model class that has a KeyProperty referencing it; in this example PhoneNumber (but there could be many).

I think the solution is to ask your users to explicitly add this link when the PhoneNumber class is created.

You can make this easy for your users by giving them a subclass of KeyProperty that takes care of this; e.g.

```
class LinkedKeyProperty(ndb.KeyProperty):
    def _fix_up(self, cls, code_name):
        super(LinkedKeyProperty, self)._fix_up(cls, code_name)
        modelclass = ndb.Model._kind_map[self._kind]
        collection_name = '%s_ref_%s_to_%s' % (cls.__name__,
                                               code_name,
                                               modelclass.__name__)
        setattr(modelclass, collection_name, (cls, self))
```

Exactly how you pick the name for the collection and the value to store there is up to you; just put something there that makes it easy for you to follow the link back. The example would create a new attribute on Contact:

```
Contact.PhoneNumber_ref_contact_to_Contact == (PhoneNumber, PhoneNumber.contact)
```

[edited to make the code working and to add an example. :-) ]

Following backreferences of unknown kinds in NDB

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