ndb modelling one-to-many: merits of repeated KeyProperty vs foreign key

Question

My question is about modelling one-to-many relations in ndb. I understand that this can be done in (at least) two different ways: with a repeated property or with a 'foreign key'. I have created a small example below. Basically we have an Article which can have an arbitrary number of Tags. Let's assume that a Tag can be removed but cannot be changed after it has been added. Let's also assume that we don't worry about transactional safety.

My question is: what is the preferred way of modelling these relationships?

My considerations:

• Approach (A) requires two writes for every tag that is added to an article (one for the Article and one for the Tag) whereas approach (B) only requires one write (just the Tag).
• Approach (A) leverages ndb's caching mechanism when fetching all Tags for an Article whereas in case of approach (B) a query is required (and additionally some custom caching)

Are there some things that I'm missing here, any other considerations that should be taken into account?

Thanks very much for your help.

Example (A):

class Article(ndb.Model):
    title = ndb.StringProperty()
    # some more properties
    tags = ndb.KeyProperty(kind="Tag", repeated=True)

    def create_tag(self):
        # requires two writes
        tag = Tag(name="my_tag")
        tag.put()
        self.tags.append(tag)
        self.put()

    def get_tags(self):
        return ndb.get_multi(self.tags)

class Tag(ndb.Model):
    name = ndb.StringProperty()
    user = ndb.KeyProperty(Kind="User") #  User that created the tag
    # some more properties

Example(B):

class Article(ndb.Model):
    title = ndb.StringProperty()
    # some more properties

    def create_tag(self):
        # requires one write
        tag = Tag(name="my_tag", article=self.key)
        tag.put()

    def get_tags(self):
        # obviously we could cache this query in memcache
        return Tag.gql("WHERE article :1", self.key)

class Tag(ndb.Model):
    name = ndb.StringProperty()
    article = ndb.KeyProperty(kind="Article")
    user = ndb.KeyProperty(Kind="User") #  User that created the tag
    # some more properties

Answer 1

Have you looked at the following about using Structured Properties https://developers.google.com/appengine/docs/python/ndb/properties#structured . The short discussion there about Contact and Addresse may simplify your problem. Also look at https://developers.google.com/appengine/docs/python/ndb/queries#filtering_structured_properties. The discussions are very short.

Also, looking ahead to the fact that joins are not allowed, option A looks better.