Google App Engine NDB Snippets

linked_key.py

# Creates a back reference to Unknown kinds in a one-to-many model (from Guido Van Rossum)
# Source: http://stackoverflow.com/questions/10731433/following-backreferences-of-unknown-kinds-in-ndb
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))

list_all_namespaces.py

# Source: https://developers.google.com/appengine/docs/python/ndb/metadata
google.appengine.ext.ndb import metadata

nses = metadata.get_namespaces()

relational_snippets.py

# Source: http://stackoverflow.com/questions/14192331/how-to-set-an-ndb-keyproperty
class Person(ndb.Expando):
    pass

class Favourite(ndb.Expando):       
    pass

class Picture(ndb.Expando):
    pass

person  = Person()
person.put()

picture = Picture()
picture.put()

fav = Favourite(
    # Personal note: As far as I can tell, parents/ancestors *specifically* 
    # MUST be in the same Namespace as children.
    # I do not believe that this is the case for non-standard relations (i.e. person, picture)
    parent=person.key, 
    person=person.key,
    picture=picture.key
)
fav.put()