collection_name for ndb.KeyProperty

Question

db.ReferenceProperty allowed for a collection_name argument that specified how the reference object R could query for the objects that contain R as a reference.

I don't see a similar argument for ndb.KeyProperty. How do folks get around that?

Answer 1

It took a little getting used to when I made the switch to ndb, but it is actually simpler. The collection_name is just syntactic sugar for a query, and you can do the query yourself:

MyEntity(ndb.Model):
    a_key = ndb.KeyProperty()

entities = MyEntity.query(MyEntity.a_key == some_key)

Below you can see a fuller example from this Google group which creates a new @property on the referenced class (the "one";; the Post in this case) instead of on the referencing class (the "many"; the Comment in this case):

OLD db:

class Post(db.Model):
    content = db.TextProperty(required = True)   
   
class Comment(db.Model):
    """ Comment object with many-to-one relationship """
    content = db.TextProperty(required = True) 
    post = db.ReferenceProperty(Post, collection_name='comment_set')

NEW ndb:

class Post(ndb.Model):
    content = ndb.TextProperty(required = True)   

    @property
    def comment_set(self):
        return Comment.query(Comment.post == self.key)

class Comment(ndb.Model):
    """ Comment object with many-to-one relationship """
    content = ndb.TextProperty(required = True) 
    post = ndb.KeyProperty(kind=Post)

If your Model classes are in separate files, you might need to change the approach to avoid circular imports

You could use a GQL Query String in the @property:

    @property
    def comment_set(self):
        return ndb.gql('SELECT * FROM Comment WHERE post = :1').bind(self.key)

Or you could use the string 'Post' for the kind argument, instead of the class Post, to avoid circular imports:

    post = ndb.KeyProperty(kind='Post')