How do I query a many to many relationship model? - Google App Engine

Question

Here are my models:

class User(db.Model):
    id = db.StringProperty(required=True)
    created = db.DateTimeProperty(auto_now_add=True)
    updated = db.DateTimeProperty(auto_now=True)
    name = db.StringProperty(required=True)
    email = db.StringProperty()

class Page(db.Model):
    id = db.StringProperty(required=True)
    created = db.DateTimeProperty(auto_now_add=True)
    updated = db.DateTimeProperty(auto_now=True)
    name = db.StringProperty(required=True)
    link = db.StringProperty(required=True)

class UserPage(db.Model): 
    user = db.ReferenceProperty(User, collection_name='pages') 
    page = db.ReferenceProperty(Page, collection_name='users')

How would I construct a query to find a users pages?

I found an article that describes a method to do it but is this the best way? http://blog.arbingersys.com/2008/04/google-app-engine-better-many-to-many.html

Answer 1

Your answer will work, but it will perform 7 calls to the datastore:

• 1 for the call to User.get_by_key_name()
• 1 for the call to UserPage...fetch()
• 5 for each dereference of x.page.id inside the loop

An alternative approach which only does 3 calls to the datastore would be something like this:

myuser = User.get_by_key_name("1") 
up = UserPage.all().filter('user =', myuser).fetch(5)
keys = [UserPage.page.get_value_for_datastore(x) for x in up]
pages = db.get(keys)
for p in pages: 
     self.response.out.write(p.id)

See http://blog.notdot.net/2010/01/ReferenceProperty-prefetching-in-App-Engine for more details.