Say I have two classes:
class A(db.Model):
class B(db.Model):
a_reference = ReferenceProperty(A)
I can now do the following:
a = A()
a.put()
b = B();
b.a_reference = a.key()
b.put()
The documentation states the following two things:
The ReferenceProperty value can be used as if it were a model instance, and the datastore entity will be fetched and the model instance created when it is first used in this way.
And later also states:
An application can explicitly db.get() the value of a ReferenceProperty (which is a Key) to test whether the referenced entity exists.
So what does that mean? The value is a key but it can be used a model instance?
If I do:
a2 = b.a_reference
a2 will be of type A, not key. Does this mean that the variable a_reference will behave like a model instance until that instance is deleted, whereafter it will return a key (pointing to a non-existing instance)?
A ReferenceProperty will always try to return an instance of the class that the stored key points to. If the referenced object has been deleted, I believe that you get back None. From the docs:
obj1 = obj2.reference
if not obj1:
# Referenced entity was deleted.
If you want to get the key that was originally stored, you can use get_value_for_datastore:
a = A()
a.put()
b = B();
b.a_reference = a.key()
b.put()
orginial_key = b.a_reference.get_value_for_datastore()