Fixed django#12203 -- Allow using ManyToManyField with through in Admin.

django/contrib/admin/checks.py

@@ -438,27 +438,13 @@ class BaseModelAdminChecks:
             return []
         else:
             try:
-                field = obj.model._meta.get_field(field_name)
+                obj.model._meta.get_field(field_name)
             except FieldDoesNotExist:
                 # If we can't find a field on the model that matches, it could
                 # be an extra field on the form.
                 return []
             else:
-                if (
+                return []
-                    isinstance(field, models.ManyToManyField)
-                    and not field.remote_field.through._meta.auto_created
-                ):
-                    return [
-                        checks.Error(
-                            "The value of '%s' cannot include the ManyToManyField "
-                            "'%s', because that field manually specifies a "
-                            "relationship model." % (label, field_name),
-                            obj=obj.__class__,
-                            id="admin.E013",
-                        )
-                    ]
-                else:
-                    return []
 
     def _check_exclude(self, obj):
         """Check that exclude is a sequence without duplicates."""
@@ -536,16 +522,6 @@ class BaseModelAdminChecks:
                 return must_be(
                     "a many-to-many field", option=label, obj=obj, id="admin.E020"
                 )
-            elif not field.remote_field.through._meta.auto_created:
-                return [
-                    checks.Error(
-                        f"The value of '{label}' cannot include the ManyToManyField "
-                        f"'{field_name}', because that field manually specifies a "
-                        f"relationship model.",
-                        obj=obj.__class__,
-                        id="admin.E013",
-                    )
-                ]
             else:
                 return []
 

django/contrib/admin/options.py

@@ -302,10 +302,6 @@ class BaseModelAdmin(metaclass=forms.MediaDefiningClass):
         """
         Get a form Field for a ManyToManyField.
         """
-        # If it uses an intermediary model that isn't auto created, don't show
-        # a field in admin.
-        if not db_field.remote_field.through._meta.auto_created:
-            return None
         db = kwargs.get("using")
 
         if "widget" not in kwargs:

docs/ref/checks.txt

@@ -660,10 +660,7 @@ with the admin site:
 * **admin.E011**: The value of ``fieldsets[n][1]`` must contain the key
   ``fields``.
 * **admin.E012**: There are duplicate field(s) in ``fieldsets[n][1]``.
-* **admin.E013**: The value of
+* **admin.E013**: (Removed)
-  ``fields[n]/filter_horizontal[n]/filter_vertical[n]/fieldsets[n][m]`` cannot
-  include the ``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).
 * **admin.E016**: The value of ``form`` must inherit from ``BaseModelForm``.

tests/admin_checks/tests.py

@@ -856,28 +856,15 @@ class SystemChecksTestCase(SimpleTestCase):
         errors = SongAdmin(Song, AdminSite()).check()
         self.assertEqual(errors, [])
 
-    def test_graceful_m2m_fail(self):
+    def test_allow_m2m_with_through(self):
-        """
-        Regression test for #12203/#12237 - Fail more gracefully when a M2M field that
-        specifies the 'through' option is included in the 'fields' or the 'fieldsets'
-        ModelAdmin options.
-        """
-
         class BookAdmin(admin.ModelAdmin):
             fields = ["authors"]
 
         errors = BookAdmin(Book, AdminSite()).check()
-        expected = [
+        expected = []
-            checks.Error(
-                "The value of 'fields' cannot include the ManyToManyField 'authors', "
-                "because that field manually specifies a relationship model.",
-                obj=BookAdmin,
-                id="admin.E013",
-            )
-        ]
         self.assertEqual(errors, expected)
 
-    def test_cannot_include_through(self):
+    def test_m2m_can_include_through(self):
         class FieldsetBookAdmin(admin.ModelAdmin):
             fieldsets = (
                 ("Header 1", {"fields": ("name",)}),
@@ -885,15 +872,7 @@ class SystemChecksTestCase(SimpleTestCase):
             )
 
         errors = FieldsetBookAdmin(Book, AdminSite()).check()
-        expected = [
+        expected = []
-            checks.Error(
-                "The value of 'fieldsets[1][1][\"fields\"]' cannot include the "
-                "ManyToManyField 'authors', because that field manually specifies a "
-                "relationship model.",
-                obj=FieldsetBookAdmin,
-                id="admin.E013",
-            )
-        ]
         self.assertEqual(errors, expected)
 
     def test_nested_fields(self):

tests/admin_widgets/models.py

@@ -201,13 +201,21 @@ class Student(models.Model):
 
 class School(models.Model):
     name = models.CharField(max_length=255)
-    students = models.ManyToManyField(Student, related_name="current_schools")
+    students = models.ManyToManyField(
+        Student, related_name="current_schools", through="StudentSchool"
+    )
     alumni = models.ManyToManyField(Student, related_name="previous_schools")
 
     def __str__(self):
         return self.name
 
 
+class StudentSchool(models.Model):
+    student = models.ForeignKey(Student, models.CASCADE)
+    school = models.ForeignKey(School, models.CASCADE)
+    extra_info = models.CharField(max_length=10)
+
+
 class Profile(models.Model):
     user = models.ForeignKey("auth.User", models.CASCADE, to_field="username")
 

tests/modeladmin/test_checks.py

@@ -376,13 +376,7 @@ class FilterVerticalCheckTests(CheckTestCase):
         class TestModelAdmin(ModelAdmin):
             filter_vertical = ["bands"]
 
-        self.assertIsInvalid(
+        self.assertIsValid(TestModelAdmin, Artist)
-            TestModelAdmin,
-            Artist,
-            "The value of 'filter_vertical[0]' cannot include the ManyToManyField "
-            "'bands', because that field manually specifies a relationship model.",
-            "admin.E013",
-        )
 
     def test_valid_case(self):
         class TestModelAdmin(ModelAdmin):
@@ -445,7 +439,7 @@ class FilterHorizontalCheckTests(CheckTestCase):
         )
 
     @isolate_apps("modeladmin")
-    def test_invalid_m2m_field_with_through(self):
+    def test_valid_m2m_field_with_through(self):
         class Artist(Model):
             bands = ManyToManyField("Band", through="BandArtist")
 
@@ -456,13 +450,7 @@ class FilterHorizontalCheckTests(CheckTestCase):
         class TestModelAdmin(ModelAdmin):
             filter_horizontal = ["bands"]
 
-        self.assertIsInvalid(
+        self.assertIsValid(TestModelAdmin, Artist)
-            TestModelAdmin,
-            Artist,
-            "The value of 'filter_horizontal[0]' cannot include the ManyToManyField "
-            "'bands', because that field manually specifies a relationship model.",
-            "admin.E013",
-        )
 
     def test_valid_case(self):
         class TestModelAdmin(ModelAdmin):