Fixed #21169 -- Reworked RelatedManager methods use default filtering

The `remove()` and `clear()` methods of the related managers created by
`ForeignKey`, `GenericForeignKey`, and `ManyToManyField` suffered from a
number of issues. Some operations ran multiple data modifying queries without
wrapping them in a transaction, and some operations didn't respect default
filtering when it was present (i.e. when the default manager on the related
model implemented a custom `get_queryset()`).

Fixing the issues introduced some backward incompatible changes:

- The implementation of `remove()` for `ForeignKey` related managers changed
  from a series of `Model.save()` calls to a single `QuerySet.update()` call.
  The change means that `pre_save` and `post_save` signals aren't called anymore.

- The `remove()` and `clear()` methods for `GenericForeignKey` related
  managers now perform bulk delete so `Model.delete()` isn't called anymore.

- The `remove()` and `clear()` methods for `ManyToManyField` related
  managers perform nested queries when filtering is involved, which may
  or may not be an issue depending on the database and the data itself.

Refs. #3871, #21174.

Thanks Anssi Kääriäinen and Tim Graham for the reviews.

django/db/models/fields/related.py

@@ -2,7 +2,7 @@ from operator import attrgetter
 
 from django.db import connection, connections, router, transaction
 from django.db.backends import utils
-from django.db.models import signals
+from django.db.models import signals, Q
 from django.db.models.fields import (AutoField, Field, IntegerField,
     PositiveIntegerField, PositiveSmallIntegerField, FieldDoesNotExist)
 from django.db.models.related import RelatedObject, PathInfo
@@ -464,14 +464,21 @@ def create_foreign_related_manager(superclass, rel_field, rel_model):
         # remove() and clear() are only provided if the ForeignKey can have a value of null.
         if rel_field.null:
             def remove(self, *objs):
+                # If there aren't any objects, there is nothing to do.
+                if not objs:
+                    return
+
                 val = rel_field.get_foreign_related_value(self.instance)
+
+                old_ids = set()
                 for obj in objs:
                     # Is obj actually part of this descriptor set?
                     if rel_field.get_local_related_value(obj) == val:
-                        setattr(obj, rel_field.name, None)
-                        obj.save()
+                        old_ids.add(obj.pk)
                     else:
                         raise rel_field.rel.to.DoesNotExist("%r is not related to %r." % (obj, self.instance))
+
+                self.filter(pk__in=old_ids).update(**{rel_field.name: None})
             remove.alters_data = True
 
             def clear(self):
@@ -536,6 +543,7 @@ def create_many_related_manager(superclass, rel):
             self.instance = instance
             self.symmetrical = symmetrical
             self.source_field = source_field
+            self.target_field = through._meta.get_field(target_field_name)
             self.source_field_name = source_field_name
             self.target_field_name = target_field_name
             self.reverse = reverse
@@ -572,6 +580,19 @@ def create_many_related_manager(superclass, rel):
             )
         do_not_call_in_templates = True
 
+        def _build_clear_filters(self, qs):
+            filters = Q(**{
+                self.source_field_name: self.related_val,
+                '%s__in' % self.target_field_name: qs
+            })
+
+            if self.symmetrical:
+                filters |= Q(**{
+                    self.target_field_name: self.related_val,
+                    '%s__in' % self.source_field_name: qs
+                })
+            return filters
+
         def get_queryset(self):
             try:
                 return self.instance._prefetched_objects_cache[self.prefetch_cache_name]
@@ -625,18 +646,20 @@ def create_many_related_manager(superclass, rel):
 
             def remove(self, *objs):
                 self._remove_items(self.source_field_name, self.target_field_name, *objs)
-
-                # If this is a symmetrical m2m relation to self, remove the mirror entry in the m2m table
-                if self.symmetrical:
-                    self._remove_items(self.target_field_name, self.source_field_name, *objs)
             remove.alters_data = True
 
         def clear(self):
-            self._clear_items(self.source_field_name)
+            db = router.db_for_write(self.through, instance=self.instance)
 
-            # If this is a symmetrical m2m relation to self, clear the mirror entry in the m2m table
-            if self.symmetrical:
-                self._clear_items(self.target_field_name)
+            signals.m2m_changed.send(sender=self.through, action="pre_clear",
+                instance=self.instance, reverse=self.reverse,
+                model=self.model, pk_set=None, using=db)
+            filters = self._build_clear_filters(self.using(db))
+            self.through._default_manager.using(db).filter(filters).delete()
+
+            signals.m2m_changed.send(sender=self.through, action="post_clear",
+                instance=self.instance, reverse=self.reverse,
+                model=self.model, pk_set=None, using=db)
         clear.alters_data = True
 
         def create(self, **kwargs):
@@ -722,56 +745,34 @@ def create_many_related_manager(superclass, rel):
             # *objs - objects to remove
 
             # If there aren't any objects, there is nothing to do.
-            if objs:
+            if not objs:
+                return
+
             # Check that all the objects are of the right type
             old_ids = set()
             for obj in objs:
                 if isinstance(obj, self.model):
-                        fk_val = self.through._meta.get_field(
-                            target_field_name).get_foreign_related_value(obj)[0]
+                    fk_val = self.target_field.get_foreign_related_value(obj)[0]
                     old_ids.add(fk_val)
                 else:
                     old_ids.add(obj)
-                # Work out what DB we're operating on
+
             db = router.db_for_write(self.through, instance=self.instance)
+
             # Send a signal to the other end if need be.
-                if self.reverse or source_field_name == self.source_field_name:
-                    # Don't send the signal when we are deleting the
-                    # duplicate data row for symmetrical reverse entries.
             signals.m2m_changed.send(sender=self.through, action="pre_remove",
                 instance=self.instance, reverse=self.reverse,
                 model=self.model, pk_set=old_ids, using=db)
-                # Remove the specified objects from the join table
-                self.through._default_manager.using(db).filter(**{
-                    source_field_name: self.related_val[0],
-                    '%s__in' % target_field_name: old_ids
-                }).delete()
-                if self.reverse or source_field_name == self.source_field_name:
-                    # Don't send the signal when we are deleting the
-                    # duplicate data row for symmetrical reverse entries.
+
+            old_vals_qs = self.using(db).filter(**{
+                '%s__in' % self.target_field.related_field.attname: old_ids})
+            filters = self._build_clear_filters(old_vals_qs)
+            self.through._default_manager.using(db).filter(filters).delete()
+
             signals.m2m_changed.send(sender=self.through, action="post_remove",
                 instance=self.instance, reverse=self.reverse,
                 model=self.model, pk_set=old_ids, using=db)
 
-        def _clear_items(self, source_field_name):
-            db = router.db_for_write(self.through, instance=self.instance)
-            # source_field_name: the PK colname in join table for the source object
-            if self.reverse or source_field_name == self.source_field_name:
-                # Don't send the signal when we are clearing the
-                # duplicate data rows for symmetrical reverse entries.
-                signals.m2m_changed.send(sender=self.through, action="pre_clear",
-                    instance=self.instance, reverse=self.reverse,
-                    model=self.model, pk_set=None, using=db)
-            self.through._default_manager.using(db).filter(**{
-                source_field_name: self.related_val
-            }).delete()
-            if self.reverse or source_field_name == self.source_field_name:
-                # Don't send the signal when we are clearing the
-                # duplicate data rows for symmetrical reverse entries.
-                signals.m2m_changed.send(sender=self.through, action="post_clear",
-                    instance=self.instance, reverse=self.reverse,
-                    model=self.model, pk_set=None, using=db)
-
     return ManyRelatedManager
 
 

docs/ref/models/querysets.txt

@@ -2132,6 +2132,8 @@ extract two field values, where only one is expected::
     inner_qs = Blog.objects.filter(name__contains='Ch').values('name', 'id')
     entries = Entry.objects.filter(blog__name__in=inner_qs)
 
+.. _nested-queries-performance:
+
 .. admonition:: Performance considerations
 
     Be cautious about using nested queries and understand your database

docs/releases/1.7.txt

@@ -574,6 +574,32 @@ a :exc:`~exceptions.ValueError` when encountering them, you will have to
 install pytz_. You may be affected by this problem if you use Django's time
 zone-related date formats or :mod:`django.contrib.syndication`.
 
+``remove()`` and ``clear()`` methods of related managers
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The ``remove()`` and ``clear()`` methods of the related managers created by
+``ForeignKey``, ``GenericForeignKey``, and ``ManyToManyField`` suffered from a
+number of issues. Some operations ran multiple data modifying queries without
+wrapping them in a transaction, and some operations didn't respect default
+filtering when it was present (i.e. when the default manager on the related
+model implemented a custom ``get_queryset()``).
+
+Fixing the issues introduced some backward incompatible changes:
+
+- The default implementation of ``remove()`` for ``ForeignKey`` related managers
+  changed from a series of ``Model.save()`` calls to a single
+  ``QuerySet.update()`` call. The change means that ``pre_save`` and
+  ``post_save`` signals aren't sent anymore.
+
+- The ``remove()`` and ``clear()`` methods for ``GenericForeignKey`` related
+  managers now perform bulk delete. The ``Model.delete()`` method isn't called
+  on each instance anymore.
+
+- The ``remove()`` and ``clear()`` methods for ``ManyToManyField`` related
+  managers perform nested queries when filtering is involved, which may or
+  may not be an issue depending on your database and your data itself.
+  See :ref:`this note <nested-queries-performance>` for more details.
+
 .. _pytz: https://pypi.python.org/pypi/pytz/
 
 Miscellaneous

tests/custom_managers/models.py

@@ -101,6 +101,22 @@ class Person(models.Model):
         return "%s %s" % (self.first_name, self.last_name)
 
 
+@python_2_unicode_compatible
+class FunPerson(models.Model):
+    first_name = models.CharField(max_length=30)
+    last_name = models.CharField(max_length=30)
+    fun = models.BooleanField(default=True)
+
+    favorite_book = models.ForeignKey('Book', null=True, related_name='fun_people_favorite_books')
+    favorite_thing_type = models.ForeignKey('contenttypes.ContentType', null=True)
+    favorite_thing_id = models.IntegerField(null=True)
+    favorite_thing = generic.GenericForeignKey('favorite_thing_type', 'favorite_thing_id')
+
+    objects = FunPeopleManager()
+
+    def __str__(self):
+        return "%s %s" % (self.first_name, self.last_name)
+
 @python_2_unicode_compatible
 class Book(models.Model):
     title = models.CharField(max_length=50)
@@ -108,10 +124,14 @@ class Book(models.Model):
     is_published = models.BooleanField(default=False)
     published_objects = PublishedBookManager()
     authors = models.ManyToManyField(Person, related_name='books')
+    fun_authors = models.ManyToManyField(FunPerson, related_name='books')
 
     favorite_things = generic.GenericRelation(Person,
         content_type_field='favorite_thing_type', object_id_field='favorite_thing_id')
 
+    fun_people_favorite_things = generic.GenericRelation(FunPerson,
+        content_type_field='favorite_thing_type', object_id_field='favorite_thing_id')
+
     def __str__(self):
         return self.title
 

tests/custom_managers/tests.py

@@ -3,7 +3,7 @@ from __future__ import unicode_literals
 from django.test import TestCase
 from django.utils import six
 
-from .models import Person, Book, Car, PersonManager, PublishedBookManager
+from .models import Person, FunPerson, Book, Car, PersonManager, PublishedBookManager
 
 
 class CustomManagerTests(TestCase):
@@ -12,10 +12,11 @@ class CustomManagerTests(TestCase):
             title="How to program", author="Rodney Dangerfield", is_published=True)
         self.b2 = Book.published_objects.create(
             title="How to be smart", author="Albert Einstein", is_published=False)
-        self.p1 = Person.objects.create(first_name="Bugs", last_name="Bunny", fun=True)
-        self.p2 = Person.objects.create(first_name="Droopy", last_name="Dog", fun=False)
 
     def test_manager(self):
+        Person.objects.create(first_name="Bugs", last_name="Bunny", fun=True)
+        droopy = Person.objects.create(first_name="Droopy", last_name="Dog", fun=False)
+
         # Test a custom `Manager` method.
         self.assertQuerysetEqual(
             Person.objects.get_fun_people(), [
@@ -66,7 +67,7 @@ class CustomManagerTests(TestCase):
 
         # The RelatedManager used on the 'books' descriptor extends the default
         # manager
-        self.assertIsInstance(self.p2.books, PublishedBookManager)
+        self.assertIsInstance(droopy.books, PublishedBookManager)
 
         # The default manager, "objects", doesn't exist, because a custom one
         # was provided.
@@ -113,78 +114,341 @@ class CustomManagerTests(TestCase):
             lambda c: c.name
         )
 
-    def test_related_manager_fk(self):
-        self.p1.favorite_book = self.b1
-        self.p1.save()
-        self.p2.favorite_book = self.b1
-        self.p2.save()
+    def test_fk_related_manager(self):
+        Person.objects.create(first_name="Bugs", last_name="Bunny", fun=True, favorite_book=self.b1)
+        Person.objects.create(first_name="Droopy", last_name="Dog", fun=False, favorite_book=self.b1)
+        FunPerson.objects.create(first_name="Bugs", last_name="Bunny", fun=True, favorite_book=self.b1)
+        FunPerson.objects.create(first_name="Droopy", last_name="Dog", fun=False, favorite_book=self.b1)
 
         self.assertQuerysetEqual(
             self.b1.favorite_books.order_by('first_name').all(), [
                 "Bugs",
                 "Droopy",
             ],
-            lambda c: c.first_name
+            lambda c: c.first_name,
+            ordered=False,
+        )
+        self.assertQuerysetEqual(
+            self.b1.fun_people_favorite_books.all(), [
+                "Bugs",
+            ],
+            lambda c: c.first_name,
+            ordered=False,
         )
         self.assertQuerysetEqual(
             self.b1.favorite_books(manager='boring_people').all(), [
                 "Droopy",
             ],
-            lambda c: c.first_name
+            lambda c: c.first_name,
+            ordered=False,
         )
         self.assertQuerysetEqual(
             self.b1.favorite_books(manager='fun_people').all(), [
                 "Bugs",
             ],
-            lambda c: c.first_name
+            lambda c: c.first_name,
+            ordered=False,
         )
 
-    def test_related_manager_gfk(self):
-        self.p1.favorite_thing = self.b1
-        self.p1.save()
-        self.p2.favorite_thing = self.b1
-        self.p2.save()
+    def test_gfk_related_manager(self):
+        Person.objects.create(first_name="Bugs", last_name="Bunny", fun=True, favorite_thing=self.b1)
+        Person.objects.create(first_name="Droopy", last_name="Dog", fun=False, favorite_thing=self.b1)
+        FunPerson.objects.create(first_name="Bugs", last_name="Bunny", fun=True, favorite_thing=self.b1)
+        FunPerson.objects.create(first_name="Droopy", last_name="Dog", fun=False, favorite_thing=self.b1)
 
         self.assertQuerysetEqual(
-            self.b1.favorite_things.order_by('first_name').all(), [
+            self.b1.favorite_things.all(), [
                 "Bugs",
                 "Droopy",
             ],
-            lambda c: c.first_name
+            lambda c: c.first_name,
+            ordered=False,
+        )
+        self.assertQuerysetEqual(
+            self.b1.fun_people_favorite_things.all(), [
+                "Bugs",
+            ],
+            lambda c: c.first_name,
+            ordered=False,
         )
         self.assertQuerysetEqual(
             self.b1.favorite_things(manager='boring_people').all(), [
                 "Droopy",
             ],
-            lambda c: c.first_name
+            lambda c: c.first_name,
+            ordered=False,
         )
         self.assertQuerysetEqual(
             self.b1.favorite_things(manager='fun_people').all(), [
                 "Bugs",
             ],
-            lambda c: c.first_name
+            lambda c: c.first_name,
+            ordered=False,
         )
 
-    def test_related_manager_m2m(self):
-        self.b1.authors.add(self.p1)
-        self.b1.authors.add(self.p2)
+    def test_m2m_related_manager(self):
+        bugs = Person.objects.create(first_name="Bugs", last_name="Bunny", fun=True)
+        self.b1.authors.add(bugs)
+        droopy = Person.objects.create(first_name="Droopy", last_name="Dog", fun=False)
+        self.b1.authors.add(droopy)
+        bugs = FunPerson.objects.create(first_name="Bugs", last_name="Bunny", fun=True)
+        self.b1.fun_authors.add(bugs)
+        droopy = FunPerson.objects.create(first_name="Droopy", last_name="Dog", fun=False)
+        self.b1.fun_authors.add(droopy)
 
         self.assertQuerysetEqual(
             self.b1.authors.order_by('first_name').all(), [
                 "Bugs",
                 "Droopy",
             ],
-            lambda c: c.first_name
+            lambda c: c.first_name,
+            ordered=False,
+        )
+        self.assertQuerysetEqual(
+            self.b1.fun_authors.order_by('first_name').all(), [
+                "Bugs",
+            ],
+            lambda c: c.first_name,
+            ordered=False,
         )
         self.assertQuerysetEqual(
             self.b1.authors(manager='boring_people').all(), [
                 "Droopy",
             ],
-            lambda c: c.first_name
+            lambda c: c.first_name,
+            ordered=False,
         )
         self.assertQuerysetEqual(
             self.b1.authors(manager='fun_people').all(), [
                 "Bugs",
             ],
-            lambda c: c.first_name
+            lambda c: c.first_name,
+            ordered=False,
+        )
+
+    def test_removal_through_default_fk_related_manager(self):
+        bugs = FunPerson.objects.create(first_name="Bugs", last_name="Bunny", fun=True, favorite_book=self.b1)
+        droopy = FunPerson.objects.create(first_name="Droopy", last_name="Dog", fun=False, favorite_book=self.b1)
+
+        self.b1.fun_people_favorite_books.remove(droopy)
+        self.assertQuerysetEqual(
+            FunPerson._base_manager.filter(favorite_book=self.b1), [
+                "Bugs",
+                "Droopy",
+            ],
+            lambda c: c.first_name,
+            ordered=False,
+        )
+
+        self.b1.fun_people_favorite_books.remove(bugs)
+        self.assertQuerysetEqual(
+            FunPerson._base_manager.filter(favorite_book=self.b1), [
+                "Droopy",
+            ],
+            lambda c: c.first_name,
+            ordered=False,
+        )
+        bugs.favorite_book = self.b1
+        bugs.save()
+
+        self.b1.fun_people_favorite_books.clear()
+        self.assertQuerysetEqual(
+            FunPerson._base_manager.filter(favorite_book=self.b1), [
+                "Droopy",
+            ],
+            lambda c: c.first_name,
+            ordered=False,
+        )
+
+    def test_removal_through_specified_fk_related_manager(self):
+        Person.objects.create(first_name="Bugs", last_name="Bunny", fun=True, favorite_book=self.b1)
+        droopy = Person.objects.create(first_name="Droopy", last_name="Dog", fun=False, favorite_book=self.b1)
+
+        # Check that the fun manager DOESN'T remove boring people.
+        self.b1.favorite_books(manager='fun_people').remove(droopy)
+        self.assertQuerysetEqual(
+            self.b1.favorite_books(manager='boring_people').all(), [
+                "Droopy",
+            ],
+            lambda c: c.first_name,
+            ordered=False,
+        )
+        # Check that the boring manager DOES remove boring people.
+        self.b1.favorite_books(manager='boring_people').remove(droopy)
+        self.assertQuerysetEqual(
+            self.b1.favorite_books(manager='boring_people').all(), [
+            ],
+            lambda c: c.first_name,
+            ordered=False,
+        )
+        droopy.favorite_book = self.b1
+        droopy.save()
+
+        # Check that the fun manager ONLY clears fun people.
+        self.b1.favorite_books(manager='fun_people').clear()
+        self.assertQuerysetEqual(
+            self.b1.favorite_books(manager='boring_people').all(), [
+                "Droopy",
+            ],
+            lambda c: c.first_name,
+            ordered=False,
+        )
+        self.assertQuerysetEqual(
+            self.b1.favorite_books(manager='fun_people').all(), [
+            ],
+            lambda c: c.first_name,
+            ordered=False,
+        )
+
+    def test_removal_through_default_gfk_related_manager(self):
+        bugs = FunPerson.objects.create(first_name="Bugs", last_name="Bunny", fun=True, favorite_thing=self.b1)
+        droopy = FunPerson.objects.create(first_name="Droopy", last_name="Dog", fun=False, favorite_thing=self.b1)
+
+        self.b1.fun_people_favorite_things.remove(droopy)
+        self.assertQuerysetEqual(
+            FunPerson._base_manager.order_by('first_name').filter(favorite_thing_id=self.b1.pk), [
+                "Bugs",
+                "Droopy",
+            ],
+            lambda c: c.first_name,
+            ordered=False,
+        )
+
+        self.b1.fun_people_favorite_things.remove(bugs)
+        self.assertQuerysetEqual(
+            FunPerson._base_manager.order_by('first_name').filter(favorite_thing_id=self.b1.pk), [
+                "Droopy",
+            ],
+            lambda c: c.first_name,
+            ordered=False,
+        )
+        bugs.favorite_book = self.b1
+        bugs.save()
+
+        self.b1.fun_people_favorite_things.clear()
+        self.assertQuerysetEqual(
+            FunPerson._base_manager.order_by('first_name').filter(favorite_thing_id=self.b1.pk), [
+                "Droopy",
+            ],
+            lambda c: c.first_name,
+            ordered=False,
+        )
+
+    def test_removal_through_specified_gfk_related_manager(self):
+        Person.objects.create(first_name="Bugs", last_name="Bunny", fun=True, favorite_thing=self.b1)
+        droopy = Person.objects.create(first_name="Droopy", last_name="Dog", fun=False, favorite_thing=self.b1)
+
+        # Check that the fun manager DOESN'T remove boring people.
+        self.b1.favorite_things(manager='fun_people').remove(droopy)
+        self.assertQuerysetEqual(
+            self.b1.favorite_things(manager='boring_people').all(), [
+                "Droopy",
+            ],
+            lambda c: c.first_name,
+            ordered=False,
+        )
+
+        # Check that the boring manager DOES remove boring people.
+        self.b1.favorite_things(manager='boring_people').remove(droopy)
+        self.assertQuerysetEqual(
+            self.b1.favorite_things(manager='boring_people').all(), [
+            ],
+            lambda c: c.first_name,
+            ordered=False,
+        )
+        droopy.favorite_thing = self.b1
+        droopy.save()
+
+        # Check that the fun manager ONLY clears fun people.
+        self.b1.favorite_things(manager='fun_people').clear()
+        self.assertQuerysetEqual(
+            self.b1.favorite_things(manager='boring_people').all(), [
+                "Droopy",
+            ],
+            lambda c: c.first_name,
+            ordered=False,
+        )
+        self.assertQuerysetEqual(
+            self.b1.favorite_things(manager='fun_people').all(), [
+            ],
+            lambda c: c.first_name,
+            ordered=False,
+        )
+
+    def test_removal_through_default_m2m_related_manager(self):
+        bugs = FunPerson.objects.create(first_name="Bugs", last_name="Bunny", fun=True)
+        self.b1.fun_authors.add(bugs)
+        droopy = FunPerson.objects.create(first_name="Droopy", last_name="Dog", fun=False)
+        self.b1.fun_authors.add(droopy)
+
+        self.b1.fun_authors.remove(droopy)
+        self.assertQuerysetEqual(
+            self.b1.fun_authors.through._default_manager.all(), [
+                "Bugs",
+                "Droopy",
+            ],
+            lambda c: c.funperson.first_name,
+            ordered=False,
+        )
+
+        self.b1.fun_authors.remove(bugs)
+        self.assertQuerysetEqual(
+            self.b1.fun_authors.through._default_manager.all(), [
+                "Droopy",
+            ],
+            lambda c: c.funperson.first_name,
+            ordered=False,
+        )
+        self.b1.fun_authors.add(bugs)
+
+        self.b1.fun_authors.clear()
+        self.assertQuerysetEqual(
+            self.b1.fun_authors.through._default_manager.all(), [
+                "Droopy",
+            ],
+            lambda c: c.funperson.first_name,
+            ordered=False,
+        )
+
+    def test_removal_through_specified_m2m_related_manager(self):
+        bugs = Person.objects.create(first_name="Bugs", last_name="Bunny", fun=True)
+        self.b1.authors.add(bugs)
+        droopy = Person.objects.create(first_name="Droopy", last_name="Dog", fun=False)
+        self.b1.authors.add(droopy)
+
+        # Check that the fun manager DOESN'T remove boring people.
+        self.b1.authors(manager='fun_people').remove(droopy)
+        self.assertQuerysetEqual(
+            self.b1.authors(manager='boring_people').all(), [
+                "Droopy",
+            ],
+            lambda c: c.first_name,
+            ordered=False,
+        )
+
+        # Check that the boring manager DOES remove boring people.
+        self.b1.authors(manager='boring_people').remove(droopy)
+        self.assertQuerysetEqual(
+            self.b1.authors(manager='boring_people').all(), [
+            ],
+            lambda c: c.first_name,
+            ordered=False,
+        )
+        self.b1.authors.add(droopy)
+
+
+        # Check that the fun manager ONLY clears fun people.
+        self.b1.authors(manager='fun_people').clear()
+        self.assertQuerysetEqual(
+            self.b1.authors(manager='boring_people').all(), [
+                "Droopy",
+            ],
+            lambda c: c.first_name,
+            ordered=False,
+        )
+        self.assertQuerysetEqual(
+            self.b1.authors(manager='fun_people').all(), [
+            ],
+            lambda c: c.first_name,
+            ordered=False,
         )