Deserialisation of child models should not validate ParentalKey

[gasman]

On a fresh bakerydemo install:

from bakerydemo.blog.models import BlogPage

BlogPage.from_json('''{
  "pk": 62,
  "blog_person_relationship": [
    {
      "pk": 10,
      "sort_order": 0,
      "page": 62,
      "people": 3
    }
  ]
}''').blog_person_relationship.all()

# returns: [<BlogPeopleRelationship: BlogPeopleRelationship object (10)>]

However:

BlogPage.from_json('''{
  "pk": 999,
  "blog_person_relationship": [
    {
      "pk": 10,
      "sort_order": 0,
      "page": 999,
      "people": 3
    }
  ]
}''').blog_person_relationship.all()

# returns: []

This is because, when the blog_person_relationship child object is deserialised, foreign keys including the ParentalKey to Page are integrity-checked according to the on_delete property. Since there is no existing Page in the database with ID 999, and the ParentalKey has on_delete=CASCADE, the deserialisation logic decides that the child object should not exist, and drops it from the relation. Clearly, this is wrong - it should be able to recognise from the fact that it's a ParentalKey that the parent object is being created at the same time as the child object, and thus skip over the check for it existing in the database.

For a ParentalKey with on_delete=PROTECT, this has a different effect: since the dangling-foreign-key check does not implement a case for PROTECT, deserialisation fails with "can't currently handle on_delete types other than CASCADE, SET_NULL and DO_NOTHING". (For a legitimate dangling foreign key, this would not be entirely a bad thing, since the logical outcome would be an IntegrityError anyway; however, in this case we ought to be skipping that check entirely.)