merge_into.py

class EverythingCollector(Collector):
    """
    More or less identical to the default Django collector except we always
    return relations (even when they shouldnt matter).
    """
    def collect(self, objs, source=None, nullable=False, collect_related=True,
                source_attr=None, reverse_dependency=False):
        new_objs = self.add(objs)
        if not new_objs:
            return

        model = new_objs[0].__class__

        # Recursively collect concrete model's parent models, but not their
        # related objects. These will be found by meta.get_all_related_objects()
        concrete_model = model._meta.concrete_model
        for ptr in concrete_model._meta.parents.iteritems():
            if ptr:
                # FIXME: This seems to be buggy and execute a query for each
                # parent object fetch. We have the parent data in the obj,
                # but we don't have a nice way to turn that data into parent
                # object instance.
                parent_objs = [getattr(obj, ptr.name) for obj in new_objs]
                self.collect(parent_objs, source=model,
                             source_attr=ptr.rel.related_name,
                             collect_related=False,
                             reverse_dependency=True)

        if collect_related:
            for related in model._meta.get_all_related_objects(
                    include_hidden=True, include_proxy_eq=True):
                sub_objs = self.related_objects(related, new_objs)
                self.add(sub_objs)

            # TODO This entire block is only needed as a special case to
            # support cascade-deletes for GenericRelation. It should be
            # removed/fixed when the ORM gains a proper abstraction for virtual
            # or composite fields, and GFKs are reworked to fit into that.
            for relation in model._meta.many_to_many:
                if not relation.rel.through:
                    sub_objs = relation.bulk_related_objects(new_objs, self.using)
                    self.collect(sub_objs,
                                 source=model,
                                 source_attr=relation.rel.related_name,
                                 nullable=True)


def merge_into(self, other, callback=lambda x: x, using='default'):
    """
    Collects objects related to ``self`` and updates their foreign keys to
    point to ``other``.

    If ``callback`` is specified, it will be executed on each collected chunk
    before any changes are made, and should return a modified list of results
    that still need updated.

    NOTE: Duplicates (unique constraints) which exist and are bound to ``other``
    are preserved, and relations on ``self`` are discarded.
    """
    # TODO: proper support for database routing
    s_model = type(self)

    # Find all the objects than need to be deleted.
    collector = EverythingCollector(using=using)
    collector.collect([self])

    for model, objects in collector.data.iteritems():
        # find all potential keys which match our type
        fields = set(
            f.name for f in model._meta.fields
            if isinstance(f, ForeignKey)
            and f.rel.to == s_model
            if f.rel.to
        )
        print model, objects, fields
        if not fields:
            # the collector pulls in the self reference, so if it's our model
            # we actually assume it's probably not related to itself, and its
            # perfectly ok
            if model == s_model:
                continue
            raise TypeError('Unable to determine related keys on %r' % model)

        for obj in objects:
            send_signals = not model._meta.auto_created

            # find fields which need changed
            update_kwargs = {}
            for f_name in fields:
                if getattr(obj, f_name) == self:
                    update_kwargs[f_name] = other

            if not update_kwargs:
                # as before, if we're referencing ourself, this is ok
                if obj == self:
                    continue
                raise ValueError('Mismatched row present in related results')

            signal_kwargs = {
                'sender': model,
                'instance': obj,
                'using': using,
                'migrated': True,
            }

            if send_signals:
                pre_delete.send(**signal_kwargs)
                post_delete.send(**signal_kwargs)

            for k, v in update_kwargs.iteritems():
                setattr(obj, k, v)

            if send_signals:
                pre_save.send(created=True, **signal_kwargs)

            sid = transaction.savepoint(using=using)

            try:
                model.objects.filter(pk=obj.pk).update(**update_kwargs)
            except IntegrityError:
                # duplicate key exists, destroy the relations
                transaction.savepoint_rollback(sid, using=using)
                model.objects.filter(pk=obj.pk).delete()
            else:
                transaction.savepoint_commit(sid, using=using)

            if send_signals:
                post_save.send(created=True, **signal_kwargs)

proof_it_works.py

class MergeIntoTest(TestCase):
    def test_all_the_things(self):
        user_1 = User.objects.create(username='original')
        user_2 = User.objects.create(username='new')
        team_1 = Team.objects.create(owner=user_1)
        team_2 = Team.objects.create(owner=user_2)
        project_1 = Project.objects.create(owner=user_1, team=team_1)
        project_2 = Project.objects.create(owner=user_2, team=team_2)
        ag = AccessGroup.objects.create(team=team_2)
        ag.members.add(user_1)
        ag.members.add(user_2)

        merge_into(user_1, user_2)

        assert Team.objects.get(id=team_1.id).owner == user_2
        assert Team.objects.get(id=team_2.id).owner == user_2
        assert Project.objects.get(id=project_1.id).owner == user_2
        assert Project.objects.get(id=project_2.id).owner == user_2
        assert list(ag.members.all()) == [user_2]

        # make sure we didnt remove the instance
        assert User.objects.filter(id=user_1.id).exists()