Fixed #22217 - ManyToManyField.through_fields fixes.

- Docs description of arguments mix up.
- Keep it from erroneously masking E332 check.
- Add checks E338 and E339, tweak message of E337.

django/db/models/fields/related.py

@@ -1904,21 +1904,7 @@ def _check_relationship_model(self, from_model=None, **kwargs):
                 )
             )
 
-        elif self.rel.through_fields is not None:
-            if not len(self.rel.through_fields) >= 2 or not (self.rel.through_fields[0] and self.rel.through_fields[1]):
-                errors.append(
-                    checks.Error(
-                        ("The field is given an iterable for through_fields, "
-                         "which does not provide the names for both link fields "
-                         "that Django should use for the relation through model "
-                         "'%s'.") % qualified_model_name,
-                        hint=None,
-                        obj=self,
-                        id='fields.E337',
-                    )
-                )
-
-        elif not isinstance(self.rel.through, six.string_types):
+        else:
 
             assert from_model is not None, \
                 "ManyToManyField with intermediate " \
@@ -2018,6 +2004,78 @@ def _check_relationship_model(self, from_model=None, **kwargs):
                             id='fields.E336',
                         )
                     )
+
+        # Validate `through_fields`
+        if self.rel.through_fields is not None:
+            # Validate that we're given an iterable of at least two items
+            # and that none of them is "falsy"
+            if not (len(self.rel.through_fields) >= 2 and
+                    self.rel.through_fields[0] and self.rel.through_fields[1]):
+                errors.append(
+                    checks.Error(
+                        ("Field specifies 'through_fields' but does not "
+                         "provide the names of the two link fields that should be "
+                         "used for the relation through model "
+                         "'%s'.") % qualified_model_name,
+                        hint=("Make sure you specify 'through_fields' as "
+                              "through_fields=('field1', 'field2')"),
+                        obj=self,
+                        id='fields.E337',
+                    )
+                )
+
+            # Validate the given through fields -- they should be actual
+            # fields on the through model, and also be foreign keys to the
+            # expected models
+            else:
+                assert from_model is not None, \
+                    "ManyToManyField with intermediate " \
+                    "tables cannot be checked if you don't pass the model " \
+                    "where the field is attached to."
+
+                source, through, target = from_model, self.rel.through, self.rel.to
+                source_field_name, target_field_name = self.rel.through_fields[:2]
+
+                for field_name, related_model in ((source_field_name, source),
+                                                  (target_field_name, target)):
+
+                    possible_field_names = []
+                    for f in through._meta.fields:
+                        if hasattr(f, 'rel') and getattr(f.rel, 'to', None) == related_model:
+                            possible_field_names.append(f.name)
+                    if possible_field_names:
+                        hint = ("Did you mean one of the following foreign "
+                                "keys to '%s': %s?") % (related_model._meta.object_name,
+                                                        ', '.join(possible_field_names))
+                    else:
+                        hint = None
+
+                    try:
+                        field = through._meta.get_field(field_name)
+                    except FieldDoesNotExist:
+                        errors.append(
+                            checks.Error(
+                                ("The intermediary model '%s' has no field '%s'.") % (
+                                    qualified_model_name, field_name),
+                                hint=hint,
+                                obj=self,
+                                id='fields.E338',
+                            )
+                        )
+                    else:
+                        if not (hasattr(field, 'rel') and
+                                getattr(field.rel, 'to', None) == related_model):
+                            errors.append(
+                                checks.Error(
+                                    "'%s.%s' is not a foreign key to '%s'." % (
+                                        through._meta.object_name, field_name,
+                                        related_model._meta.object_name),
+                                    hint=hint,
+                                    obj=self,
+                                    id='fields.E339',
+                                )
+                            )
+
         return errors
 
     def deconstruct(self):
@@ -2104,9 +2162,6 @@ def _get_m2m_attr(self, related, attr):
                     (link_field_name is None or link_field_name == f.name):
                 setattr(self, cache_attr, getattr(f, attr))
                 return getattr(self, cache_attr)
-        # We only reach here if we're given an invalid field name via the
-        # `through_fields` argument
-        raise FieldDoesNotExist(link_field_name)
 
     def _get_m2m_reverse_attr(self, related, attr):
         "Function that can be curried to provide the related accessor or DB column name for the m2m table"
@@ -2133,12 +2188,7 @@ def _get_m2m_reverse_attr(self, related, attr):
                 elif link_field_name is None or link_field_name == f.name:
                     setattr(self, cache_attr, getattr(f, attr))
                     break
-        try:
-            return getattr(self, cache_attr)
-        except AttributeError:
-            # We only reach here if we're given an invalid reverse field
-            # name via the `through_fields` argument
-            raise FieldDoesNotExist(link_field_name)
+        return getattr(self, cache_attr)
 
     def value_to_string(self, obj):
         data = ''

docs/ref/checks.txt

@@ -94,7 +94,9 @@ Related Fields
 * **fields.E334**: The model is used as an intermediate model by ``<model>``, but it has more than one foreign key from ``<model>``, which is ambiguous. You must specify which foreign key Django should use via the through_fields keyword argument.
 * **fields.E335**: The model is used as an intermediate model by ``<model>``, but it has more than one foreign key to ``<model>``, which is ambiguous. You must specify which foreign key Django should use via the through_fields keyword argument.
 * **fields.E336**: The model is used as an intermediary model by ``<model>``, but it does not have foreign key to ``<model>`` or ``<model>``.
-* **fields.E337**: The field is given an iterable for through_fields, which does not provide the names for both link fields that Django should use for the relation through <model>.
+* **fields.E337**: Field specifies ``through_fields`` but does not provide the names of the two link fields that should be used for the relation through ``<model>``.
+* **fields.E338**: The intermediary model ``<through model>`` has no field ``<field name>``.
+* **fields.E339**: ``<model>.<field name>`` is not a foreign key to ``<model>``.
 
 Signals
 ~~~~~~~

docs/ref/models/fields.txt

@@ -1353,11 +1353,11 @@ that control how the relationship functions.
 
         class Group(models.Model):
             name = models.CharField(max_length=128)
-            members = models.ManyToManyField(Person, through='Membership', through_fields=('person', 'group'))
+            members = models.ManyToManyField(Person, through='Membership', through_fields=('group', 'person'))
 
         class Membership(models.Model):
-            person = models.ForeignKey(Person)
             group = models.ForeignKey(Group)
+            person = models.ForeignKey(Person)
             inviter = models.ForeignKey(Person, related_name="membership_invites")
             invite_reason = models.CharField(max_length=64)
 
@@ -1368,9 +1368,10 @@ that control how the relationship functions.
     above.
 
     ``through_fields`` accepts a 2-tuple ``('field1', 'field2')``, where
-    ``field1`` is the name of the foreign key to the target model (``person``
-    in this case), and ``field2`` the name of the foreign key to the model the
-    :class:`ManyToManyField` is defined on (``group`` in this case).
+    ``field1`` is the name of the foreign key to the model the
+    :class:`ManyToManyField` is defined on (``group`` in this case), and
+    ``field2`` the name of the foreign key to the target model (``person``
+    in this case).
 
     When you have more than one foreign key on an intermediary model to any
     (or even both) of the models participating in a many-to-many relationship,

docs/topics/db/models.txt

@@ -440,12 +440,12 @@ explicit declaration defines how the two models are related.
 There are a few restrictions on the intermediate model:
 
 * Your intermediate model must contain one - and *only* one - foreign key
-  to the target model (this would be ``Person`` in our example), or you must
+  to the source model (this would be ``Group`` in our example), or you must
   explicitly specify the foreign keys Django should use for the relationship
   using :attr:`ManyToManyField.through_fields <ManyToManyField.through_fields>`.
   If you have more than one foreign key and ``through_fields`` is not
   specified, a validation error will be raised. A similar restriction applies
-  to the foreign key to the source model (this would be ``Group`` in our
+  to the foreign key to the target model (this would be ``Person`` in our
   example).
 
 * For a model which has a many-to-many relationship to itself through an

tests/invalid_models_tests/test_relative_fields.py

@@ -240,6 +240,32 @@ class Relationship(models.Model):
         ]
         self.assertEqual(errors, expected)
 
+    def test_symmetric_self_reference_with_intermediate_table_and_through_fields(self):
+        """Using through_fields in a m2m with an intermediate model shouldn't mask its incompatibility with symmetry."""
+        class Person(models.Model):
+            # Explicit symmetrical=True.
+            friends = models.ManyToManyField('self',
+                symmetrical=True,
+                through="Relationship",
+                through_fields=('first', 'second'))
+
+        class Relationship(models.Model):
+            first = models.ForeignKey(Person, related_name="rel_from_set")
+            second = models.ForeignKey(Person, related_name="rel_to_set")
+            referee = models.ForeignKey(Person, related_name="referred")
+
+        field = Person._meta.get_field('friends')
+        errors = field.check(from_model=Person)
+        expected = [
+            Error(
+                'Many-to-many fields with intermediate tables must not be symmetrical.',
+                hint=None,
+                obj=field,
+                id='fields.E332',
+            ),
+        ]
+        self.assertEqual(errors, expected)
+
     def test_foreign_key_to_abstract_model(self):
         class Model(models.Model):
             foreign_key = models.ForeignKey('AbstractModel')
@@ -1071,43 +1097,69 @@ class Fan(models.Model):
             ValueError, 'Cannot specify through_fields without a through model',
             models.ManyToManyField, Fan, through_fields=('f1', 'f2'))
 
-    def test_invalid_m2m_field(self):
+    def test_invalid_order(self):
         """
-        Tests that providing invalid source field name to ManyToManyField.through_fields
-        raises FieldDoesNotExist.
+        Tests that mixing up the order of link fields to ManyToManyField.through_fields
+        triggers validation errors.
         """
         class Fan(models.Model):
             pass
 
         class Event(models.Model):
-            invitees = models.ManyToManyField(Fan, through='Invitation', through_fields=('invalid_field', 'invitee'))
+            invitees = models.ManyToManyField(Fan, through='Invitation', through_fields=('invitee', 'event'))
 
         class Invitation(models.Model):
             event = models.ForeignKey(Event)
             invitee = models.ForeignKey(Fan)
             inviter = models.ForeignKey(Fan, related_name='+')
 
-        with self.assertRaisesMessage(FieldDoesNotExist, 'invalid_field'):
-            Event().invitees.all()
+        field = Event._meta.get_field('invitees')
+        errors = field.check(from_model=Event)
+        expected = [
+            Error(
+                ("'Invitation.invitee' is not a foreign key to 'Event'."),
+                hint="Did you mean one of the following foreign keys to 'Event': event?",
+                obj=field,
+                id='fields.E339'),
+            Error(
+                ("'Invitation.event' is not a foreign key to 'Fan'."),
+                hint="Did you mean one of the following foreign keys to 'Fan': invitee, inviter?",
+                obj=field,
+                id='fields.E339'),
+        ]
+        self.assertEqual(expected, errors)
 
-    def test_invalid_m2m_reverse_field(self):
+    def test_invalid_field(self):
         """
-        Tests that providing invalid reverse field name to ManyToManyField.through_fields
-        raises FieldDoesNotExist.
+        Tests that providing invalid field names to ManyToManyField.through_fields
+        triggers validation errors.
         """
         class Fan(models.Model):
             pass
 
         class Event(models.Model):
-            invitees = models.ManyToManyField(Fan, through='Invitation', through_fields=('event', 'invalid_field'))
+            invitees = models.ManyToManyField(Fan, through='Invitation', through_fields=('invalid_field_1', 'invalid_field_2'))
 
         class Invitation(models.Model):
             event = models.ForeignKey(Event)
             invitee = models.ForeignKey(Fan)
             inviter = models.ForeignKey(Fan, related_name='+')
 
-        with self.assertRaisesMessage(FieldDoesNotExist, 'invalid_field'):
-            Event().invitees.all()
+        field = Event._meta.get_field('invitees')
+        errors = field.check(from_model=Event)
+        expected = [
+            Error(
+                ("The intermediary model 'invalid_models_tests.Invitation' has no field 'invalid_field_1'."),
+                hint="Did you mean one of the following foreign keys to 'Event': event?",
+                obj=field,
+                id='fields.E338'),
+            Error(
+                ("The intermediary model 'invalid_models_tests.Invitation' has no field 'invalid_field_2'."),
+                hint="Did you mean one of the following foreign keys to 'Fan': invitee, inviter?",
+                obj=field,
+                id='fields.E338'),
+        ]
+        self.assertEqual(expected, errors)
 
     def test_explicit_field_names(self):
         """
@@ -1129,11 +1181,11 @@ class Invitation(models.Model):
         errors = field.check(from_model=Event)
         expected = [
             Error(
-                ("The field is given an iterable for through_fields, "
-                 "which does not provide the names for both link fields "
-                 "that Django should use for the relation through model "
-                 "'invalid_models_tests.Invitation'."),
-                hint=None,
+                ("Field specifies 'through_fields' but does not provide the names "
+                 "of the two link fields that should be used for the relation "
+                 "through model 'invalid_models_tests.Invitation'."),
+                hint=("Make sure you specify 'through_fields' as "
+                      "through_fields=('field1', 'field2')"),
                 obj=field,
                 id='fields.E337')]
         self.assertEqual(expected, errors)