[1.10.x] Fixed #26998 -- Reverted some admin checks from checking field.many_to_many back to isinstance(field, models.ManyToManyField).

This partially reverts 983c158da7

Backport of 384f89f8f8 from master

django/contrib/admin/checks.py

@@ -217,10 +217,11 @@ class BaseModelAdminChecks(object):
                 # be an extra field on the form.
                 return []
             else:
-                if field.many_to_many and not field.remote_field.through._meta.auto_created:
+                if (isinstance(field, models.ManyToManyField) and
+                        not field.remote_field.through._meta.auto_created):
                     return [
                         checks.Error(
-                            "The value of '%s' cannot include the many-to-many field '%s' "
+                            "The value of '%s' cannot include the ManyToManyField '%s', "
                             "because that field manually specifies a relationship model."
                             % (label, field_name),
                             obj=obj.__class__,
@@ -393,11 +394,11 @@ class BaseModelAdminChecks(object):
             return refer_to_missing_field(field=field_name, option=label,
                                           model=model, obj=obj, id='admin.E027')
         else:
-            if field.many_to_many or isinstance(field, (models.DateTimeField, models.ForeignKey)):
+            if isinstance(field, (models.DateTimeField, models.ForeignKey, models.ManyToManyField)):
                 return [
                     checks.Error(
                         "The value of '%s' refers to '%s', which must not be a DateTimeField, "
-                        "a foreign key, or a many-to-many field." % (label, field_name),
+                        "a ForeignKey, or a ManyToManyField." % (label, field_name),
                         obj=obj.__class__,
                         id='admin.E028',
                     )
@@ -621,10 +622,10 @@ class ModelAdminChecks(BaseModelAdminChecks):
                         id='admin.E108',
                     )
                 ]
-            elif getattr(field, 'many_to_many', False):
+            elif isinstance(field, models.ManyToManyField):
                 return [
                     checks.Error(
-                        "The value of '%s' must not be a many-to-many field." % label,
+                        "The value of '%s' must not be a ManyToManyField." % label,
                         obj=obj.__class__,
                         id='admin.E109',
                     )

docs/ref/checks.txt

@@ -302,7 +302,7 @@ with the admin site:
   ``fields``.
 * **admin.E012**: There are duplicate field(s) in ``fieldsets[n][1]``.
 * **admin.E013**: ``fields[n]/fieldsets[n][m]`` cannot include the
-  many-to-many field ``<field name>``, because that field manually specifies a
+  ``ManyToManyField`` ``<field name>``, because that field manually specifies a
   relationship model.
 * **admin.E014**: The value of ``exclude`` must be a list or tuple.
 * **admin.E015**: The value of ``exclude`` contains duplicate field(s).
@@ -326,8 +326,8 @@ with the admin site:
 * **admin.E027**: The value of ``prepopulated_fields`` refers to
   ``<field name>``, which is not an attribute of ``<model>``.
 * **admin.E028**: The value of ``prepopulated_fields`` refers to
-  ``<field name>``, which must not be a ``DateTimeField``, a foreign key or a
+  ``<field name>``, which must not be a ``DateTimeField``, a ``ForeignKey``, or a
-  many-to-many field.
+  ``ManyToManyField`` field.
 * **admin.E029**: The value of ``prepopulated_fields[<field name>]`` must be a
   list or tuple.
 * **admin.E030**: The value of ``prepopulated_fields`` refers to
@@ -361,7 +361,7 @@ with the admin site:
   which is not a callable, an attribute of ``<ModelAdmin class>``, or an
   attribute or method on ``<model>``.
 * **admin.E109**: The value of ``list_display[n]`` must not be a
-  many-to-many field.
+  ``ManyToManyField`` field.
 * **admin.E110**: The value of ``list_display_links`` must be a list, a tuple,
   or ``None``.
 * **admin.E111**: The value of ``list_display_links[n]`` refers to ``<label>``,

docs/releases/1.10.1.txt

@@ -61,3 +61,7 @@ Bugfixes
 
 * Fixed ``contrib.admindocs`` crash when a view is in a class, such as some of
   the admin views (:ticket:`27018`).
+
+* Reverted a few admin checks that checked ``field.many_to_many`` back to
+  ``isinstance(field, models.ManyToManyField)`` since it turned out the checks
+  weren't suitable to be generalized like that (:ticket:`26998`).

tests/admin_checks/tests.py

@@ -583,7 +583,7 @@ class SystemChecksTestCase(SimpleTestCase):
         errors = BookAdmin(Book, AdminSite()).check()
         expected = [
             checks.Error(
-                "The value of 'fields' cannot include the many-to-many field 'authors' "
+                "The value of 'fields' cannot include the ManyToManyField 'authors', "
                 "because that field manually specifies a relationship model.",
                 obj=BookAdmin,
                 id='admin.E013',
@@ -601,8 +601,8 @@ class SystemChecksTestCase(SimpleTestCase):
         errors = FieldsetBookAdmin(Book, AdminSite()).check()
         expected = [
             checks.Error(
-                "The value of 'fieldsets[1][1][\"fields\"]' cannot include the many-to-many field "
+                "The value of 'fieldsets[1][1][\"fields\"]' cannot include the ManyToManyField "
-                "'authors' because that field manually specifies a relationship model.",
+                "'authors', because that field manually specifies a relationship model.",
                 obj=FieldsetBookAdmin,
                 id='admin.E013',
             )

tests/modeladmin/tests.py

@@ -925,7 +925,7 @@ class PrepopulatedFieldsCheckTests(CheckTestCase):
         self.assertIsInvalid(
             ValidationTestModelAdmin, ValidationTestModel,
             ("The value of 'prepopulated_fields' refers to 'users', which must not be "
-             "a DateTimeField, a foreign key, or a many-to-many field."),
+             "a DateTimeField, a ForeignKey, or a ManyToManyField."),
             'admin.E028')
 
     def test_valid_case(self):
@@ -963,7 +963,7 @@ class ListDisplayTests(CheckTestCase):
 
         self.assertIsInvalid(
             ValidationTestModelAdmin, ValidationTestModel,
-            "The value of 'list_display[0]' must not be a many-to-many field.",
+            "The value of 'list_display[0]' must not be a ManyToManyField.",
             'admin.E109')
 
     def test_valid_case(self):