Fixed #19671 -- Added warnings that null and validators are ignored for ManyToManyField.

Thanks Loic Bistuer and Tim Graham for help and review.

django/contrib/admin/filters.py

@@ -8,6 +8,7 @@ certain test -- e.g. being a DateField or ForeignKey.
 import datetime
 
 from django.db import models
+from django.db.models.fields.related import ManyToManyField
 from django.core.exceptions import ImproperlyConfigured, ValidationError
 from django.utils.encoding import smart_text, force_text
 from django.utils.translation import ugettext_lazy as _
@@ -207,8 +208,8 @@ class RelatedFieldListFilter(FieldListFilter):
                 'display': val,
             }
         if (isinstance(self.field, models.related.RelatedObject) and
-                self.field.field.null or hasattr(self.field, 'rel') and
-                self.field.null):
+                (self.field.field.null or isinstance(self.field.field, ManyToManyField)) or
+                hasattr(self.field, 'rel') and (self.field.null or isinstance(self.field, ManyToManyField))):
             yield {
                 'selected': bool(self.lookup_val_isnull),
                 'query_string': cl.get_query_string({

django/db/models/fields/related.py

@@ -1899,6 +1899,7 @@ class ManyToManyField(RelatedField):
         errors = super(ManyToManyField, self).check(**kwargs)
         errors.extend(self._check_unique(**kwargs))
         errors.extend(self._check_relationship_model(**kwargs))
+        errors.extend(self._check_ignored_options(**kwargs))
         return errors
 
     def _check_unique(self, **kwargs):
@@ -1913,6 +1914,31 @@ class ManyToManyField(RelatedField):
             ]
         return []
 
+    def _check_ignored_options(self, **kwargs):
+        warnings = []
+
+        if self.null:
+            warnings.append(
+                checks.Warning(
+                    'null has no effect on ManyToManyField.',
+                    hint=None,
+                    obj=self,
+                    id='fields.W340',
+                )
+            )
+
+        if len(self._validators) > 0:
+            warnings.append(
+                checks.Warning(
+                    'ManyToManyField does not support validators.',
+                    hint=None,
+                    obj=self,
+                    id='fields.W341',
+                )
+            )
+
+        return warnings
+
     def _check_relationship_model(self, from_model=None, **kwargs):
         if hasattr(self.rel.through, '_meta'):
             qualified_model_name = "%s.%s" % (

docs/ref/checks.txt

@@ -148,6 +148,8 @@ Related Fields
 * **fields.E338**: The intermediary model ``<through model>`` has no field
   ``<field name>``.
 * **fields.E339**: ``<model>.<field name>`` is not a foreign key to ``<model>``.
+* **fields.W340**: ``null`` has no effect on ``ManyToManyField``.
+* **fields.W341**: ``ManyToManyField`` does not support ``validators``.
 
 Signals
 ~~~~~~~

docs/ref/models/fields.txt

@@ -1448,6 +1448,10 @@ that control how the relationship functions.
 
     If in doubt, leave it to its default of ``True``.
 
+:class:`ManyToManyField` does not support :attr:`~Field.validators`.
+
+:attr:`~Field.null` has no effect since there is no way to require a
+relationship at the database level.
 
 .. _ref-onetoone:
 

tests/admin_filters/models.py

@@ -10,7 +10,7 @@ class Book(models.Model):
     title = models.CharField(max_length=50)
     year = models.PositiveIntegerField(null=True, blank=True)
     author = models.ForeignKey(User, verbose_name="Verbose Author", related_name='books_authored', blank=True, null=True)
-    contributors = models.ManyToManyField(User, verbose_name="Verbose Contributors", related_name='books_contributed', blank=True, null=True)
+    contributors = models.ManyToManyField(User, verbose_name="Verbose Contributors", related_name='books_contributed', blank=True)
     is_best_seller = models.NullBooleanField(default=0)
     date_registered = models.DateField(null=True)
     no = models.IntegerField(verbose_name='number', blank=True, null=True)  # This field is intentionally 2 characters long. See #16080.

tests/admin_widgets/models.py

@@ -63,7 +63,7 @@ class Inventory(models.Model):
 
 class Event(models.Model):
     main_band = models.ForeignKey(Band, limit_choices_to=models.Q(pk__gt=0), related_name='events_main_band_at')
-    supporting_bands = models.ManyToManyField(Band, null=True, blank=True, related_name='events_supporting_band_at')
+    supporting_bands = models.ManyToManyField(Band, blank=True, related_name='events_supporting_band_at')
     start_date = models.DateField(blank=True, null=True)
     start_time = models.TimeField(blank=True, null=True)
     description = models.TextField(blank=True)

tests/forms_tests/models.py

@@ -92,7 +92,7 @@ class ChoiceFieldModel(models.Model):
 class OptionalMultiChoiceModel(models.Model):
     multi_choice = models.ManyToManyField(ChoiceOptionModel, blank=False, related_name='not_relevant',
                                           default=lambda: ChoiceOptionModel.objects.filter(name='default'))
-    multi_choice_optional = models.ManyToManyField(ChoiceOptionModel, blank=True, null=True,
+    multi_choice_optional = models.ManyToManyField(ChoiceOptionModel, blank=True,
                                                    related_name='not_relevant2')
 
 

tests/invalid_models_tests/test_relative_fields.py

@@ -1,7 +1,7 @@
 # -*- encoding: utf-8 -*-
 from __future__ import unicode_literals
 
-from django.core.checks import Error
+from django.core.checks import Error, Warning as DjangoWarning
 from django.db import models
 from django.test.utils import override_settings
 from django.test.testcases import skipIfDBFeature
@@ -60,6 +60,35 @@ class RelativeFieldTests(IsolatedModelsTestCase):
         ]
         self.assertEqual(errors, expected)
 
+    def test_many_to_many_with_useless_options(self):
+        class Model(models.Model):
+            name = models.CharField(max_length=20)
+
+        class ModelM2M(models.Model):
+            m2m = models.ManyToManyField(Model, null=True, validators=[''])
+
+        errors = ModelM2M.check()
+        field = ModelM2M._meta.get_field('m2m')
+
+        expected = [
+            DjangoWarning(
+                'null has no effect on ManyToManyField.',
+                hint=None,
+                obj=field,
+                id='fields.W340',
+            )
+        ]
+        expected.append(
+            DjangoWarning(
+                'ManyToManyField does not support validators.',
+                hint=None,
+                obj=field,
+                id='fields.W341',
+            )
+        )
+
+        self.assertEqual(errors, expected)
+
     def test_ambiguous_relationship_model(self):
 
         class Person(models.Model):

tests/m2m_regress/models.py

@@ -60,7 +60,7 @@ class Line(models.Model):
 
 class Worksheet(models.Model):
     id = models.CharField(primary_key=True, max_length=100)
-    lines = models.ManyToManyField(Line, blank=True, null=True)
+    lines = models.ManyToManyField(Line, blank=True)
 
 
 # Regression for #11226 -- A model with the same name that another one to

tests/model_package/models/article.py

@@ -5,4 +5,4 @@ from django.db import models
 class Article(models.Model):
     sites = models.ManyToManyField(Site)
     headline = models.CharField(max_length=100)
-    publications = models.ManyToManyField("model_package.Publication", null=True, blank=True,)
+    publications = models.ManyToManyField("model_package.Publication", blank=True)

tests/model_package/tests.py

@@ -10,9 +10,7 @@ from .models.article import Article
 
 class Advertisement(models.Model):
     customer = models.CharField(max_length=100)
-    publications = models.ManyToManyField(
-        "model_package.Publication", null=True, blank=True
-    )
+    publications = models.ManyToManyField("model_package.Publication", blank=True)
 
 
 class ModelPackageTests(TestCase):

tests/queries/models.py

@@ -101,7 +101,7 @@ class Item(models.Model):
     name = models.CharField(max_length=10)
     created = models.DateTimeField()
     modified = models.DateTimeField(blank=True, null=True)
-    tags = models.ManyToManyField(Tag, blank=True, null=True)
+    tags = models.ManyToManyField(Tag, blank=True)
     creator = models.ForeignKey(Author)
     note = models.ForeignKey(Note)
 

tests/serializers_regress/models.py

@@ -168,7 +168,7 @@ class FKDataNaturalKey(models.Model):
 
 
 class M2MData(models.Model):
-    data = models.ManyToManyField(Anchor, null=True)
+    data = models.ManyToManyField(Anchor)
 
 
 class O2OData(models.Model):
@@ -181,7 +181,7 @@ class FKSelfData(models.Model):
 
 
 class M2MSelfData(models.Model):
-    data = models.ManyToManyField('self', null=True, symmetrical=False)
+    data = models.ManyToManyField('self', symmetrical=False)
 
 
 class FKDataToField(models.Model):
@@ -193,7 +193,7 @@ class FKDataToO2O(models.Model):
 
 
 class M2MIntermediateData(models.Model):
-    data = models.ManyToManyField(Anchor, null=True, through='Intermediate')
+    data = models.ManyToManyField(Anchor, through='Intermediate')
 
 
 class Intermediate(models.Model):

tests/update_only_fields/models.py

@@ -25,7 +25,7 @@ class Person(models.Model):
 class Employee(Person):
     employee_num = models.IntegerField(default=0)
     profile = models.ForeignKey('Profile', related_name='profiles', null=True)
-    accounts = models.ManyToManyField('Account', related_name='employees', blank=True, null=True)
+    accounts = models.ManyToManyField('Account', related_name='employees', blank=True)
 
 
 @python_2_unicode_compatible