Question

I have a datastore object that uses one of the fields in the class to create the key. Suppose I have an object ‘a’ with a value ‘k’ for this field. If I try to add an object ‘b’ to the datastore, which also has value ‘k’ for the field, object ‘b’ overwrites object ‘a’. Just wanted to confirm if this is expected behaviour. While it looks quite obvious, it would be great if someone can explain the underlying concept..

Answer 1

As Adam has explained, the short answer is that two entities can’t share the same key (imagine a dict). The long answer though, is a little bit more complex. See the docs:

Paths and Key Uniqueness

The complete key of an entity,
including the path, the kind and the
name or numeric ID, is unique and
specific to that entity. The complete
key is assigned when the entity is
created in the datastore, and none of
its parts can change.

The keys of two different entities can
have similar parts as long as at least
one part is different. For instance,
two entities can have the same kind
and name if they have different
parents. Similarly, two entities can
have the same parent (or no parent)
and name if they are of different
kinds.

An application should not rely on
numeric IDs being assigned in
increasing order with the order of
entity creation. This is generally the
case, but not guaranteed.