App Engine db.model reference question

Question

How can I get at the Labels data from within my Task model?

class Task(db.Model):
  title = db.StringProperty()

class Label(db.Model):
  name = db.StringProperty()

class Tasklabel(db.Model):
  task = db.ReferenceProperty(Task)
  label = db.ReferenceProperty(Label)

creating the association is no problem, but how can I get at the labels associated with a task like:

task = Task.get('...')
for label in task.labels

Answer 1

Don't you want a ListProperty on Task like this to do a many-to-many?

class Label(db.Model)
   name = db.StringProperty()

   @property
   def members(self):
      return Task.gql("WHERE labels = :1", self.key())

class Task(db.Model)
   title = db.StringProperty();
   labels = db.ListProperty(db.Key)

Then you could do

foo_label = Label.gql("WHERE name = 'foo'").get()
task1 = Task.gql("WHERE title = 'task 1'").get()
if foo_label.key() not in task1.labels:
  task1.labels.append(foo_label.key())
task1.put()

There's a thorough article about modeling entity relationships on Google code. I stole the code above from this article.

Answer 2

This worked for me with your current datamodel:

taskObject = db.Query(Task).get()
for item in taskObject.tasklabel_set:
        item.label.name

Or you could remove the Label class and just do a one-to-many relationship between Task and TaskLabel:

class Task(db.Model):
        title = db.StringProperty()

class TaskLabel(db.Model):
        task = db.ReferenceProperty(Task)
        label = db.StringProperty()

Then

taskObject = db.Query(Task).get()
for item in taskObject.tasklabel_set:
        item.label

Here is a tip from the Google article on modeling relationships in the datastore

By defining it as a ReferenceProperty, you have created a property that can only be assigned values of type 'Task'. Every time you define a reference property, it creates an implicit collection property on the referenced class. By default, this collection is called _set. In this case, it would make a property Task.tasklabel_set.

The article can be found here.

I also recommend playing around with this code in the interactive console on the dev appserver.