ModelState should be inspected to determine whether an object is being created or updated

[jeremystretch]

Proposed Changes

Anywhere it's necessary to determine whether an object is being created or updated (e.g. inside clean()), we should inspect the instance's _state attribute, which is a ModelState object. The _adding boolean of this object indicates whether the instance is being added to the database or updated.

Justification

In many places around the application, we use if self.pk as a shorthand for determining whether an object has already been created in the database. (If not, its primary key will not yet be set.) However, this assumes that the primary key can never be known in advance, which may not necessarily be true. For instance, the netbox-branching plugin copies predetermined primary keys when creating an object.