[1.7.x] Fixed #24236 -- Treated inherited m2m fields as such if they don't define get_internal_type()

Regression introduced in 3d4a826174b7a411a03be39725e60c940944a7fe

Thanks IRC user ris for the report, Loïc Bistuer, Anssi Kääriäinen and
Andriy Sokolovskiy for the discussion and Tim Graham for the review.

3 files changed, 15 insertions, 6 deletions

diff --git a/django/db/backends/schema.py b/django/db/backends/schema.py
index 12e2ab7657..54e6e06c4f 100644
--- a/django/db/backends/schema.py
+++ b/django/db/backends/schema.py
@@ -3,6 +3,7 @@ import operator
 
 from django.db.backends.creation import BaseDatabaseCreation
 from django.db.backends.utils import truncate_name
+from django.db.models.fields.related import ManyToManyField
 from django.db.transaction import atomic
 from django.utils.encoding import force_bytes
 from django.utils.log import getLogger
@@ -358,7 +359,8 @@ class BaseDatabaseSchemaEditor(object):
         table instead (for M2M fields)
         """
         # Special-case implicit M2M tables
-        if field.get_internal_type() == 'ManyToManyField' and field.rel.through._meta.auto_created:
+        if ((isinstance(field, ManyToManyField) or field.get_internal_type() == 'ManyToManyField') and
+                field.rel.through._meta.auto_created):
             return self.create_model(field.rel.through)
         # Get the column's definition
         definition, params = self.column_sql(model, field, include_default=True)
@@ -402,7 +404,8 @@ class BaseDatabaseSchemaEditor(object):
         but for M2Ms may involve deleting a table.
         """
         # Special-case implicit M2M tables
-        if field.get_internal_type() == 'ManyToManyField' and field.rel.through._meta.auto_created:
+        if ((isinstance(field, ManyToManyField) or field.get_internal_type() == 'ManyToManyField') and
+                field.rel.through._meta.auto_created):
             return self.delete_model(field.rel.through)
         # It might not actually have a column behind it
         if field.db_parameters(connection=self.connection)['type'] is None:
diff --git a/django/db/backends/sqlite3/schema.py b/django/db/backends/sqlite3/schema.py
index c5f56a04a7..0f41cb73f9 100644
--- a/django/db/backends/sqlite3/schema.py
+++ b/django/db/backends/sqlite3/schema.py
@@ -4,6 +4,7 @@ from decimal import Decimal
 from django.utils import six
 from django.apps.registry import Apps
 from django.db.backends.schema import BaseDatabaseSchemaEditor
+from django.db.models.fields.related import ManyToManyField
 
 
 class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
@@ -69,7 +70,7 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         for field in create_fields:
             body[field.name] = field
             # Choose a default and insert it into the copy map
-            if not field.get_internal_type() == 'ManyToManyField':
+            if not (isinstance(field, ManyToManyField) or field.get_internal_type() == 'ManyToManyField'):
                 mapping[field.column] = self.quote_value(
                     self.effective_default(field)
                 )
@@ -92,7 +93,8 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
             del body[field.name]
             del mapping[field.column]
             # Remove any implicit M2M tables
-            if field.get_internal_type() == 'ManyToManyField' and field.rel.through._meta.auto_created:
+            if ((isinstance(field, ManyToManyField) or field.get_internal_type() == 'ManyToManyField') and
+                    field.rel.through._meta.auto_created):
                 return self.delete_model(field.rel.through)
         # Work inside a new app registry
         apps = Apps()
@@ -171,7 +173,8 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         table instead (for M2M fields)
         """
         # Special-case implicit M2M tables
-        if field.get_internal_type() == 'ManyToManyField' and field.rel.through._meta.auto_created:
+        if ((isinstance(field, ManyToManyField) or field.get_internal_type() == 'ManyToManyField') and
+                field.rel.through._meta.auto_created):
             return self.create_model(field.rel.through)
         self._remake_table(model, create_fields=[field])
 
@@ -181,7 +184,7 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         but for M2Ms may involve deleting a table.
         """
         # M2M fields are a special case
-        if field.get_internal_type() == 'ManyToManyField':
+        if isinstance(field, ManyToManyField) or field.get_internal_type() == 'ManyToManyField':
             # For implicit M2M tables, delete the auto-created table
             if field.rel.through._meta.auto_created:
                 self.delete_model(field.rel.through)
diff --git a/docs/releases/1.7.5.txt b/docs/releases/1.7.5.txt
index 2039c9dc4e..afce510aae 100644
--- a/docs/releases/1.7.5.txt
+++ b/docs/releases/1.7.5.txt
@@ -13,3 +13,6 @@ Bugfixes
   ``contrib.contenttypes``’s or ``contrib.auth``’s first migration
   (:ticket:`24075`) due to severe impact on the test performance
   (:ticket:`24251`) and problems in multi-database setups (:ticket:`24298`).
+
+* Fixed a regression that prevented custom fields inheriting from
+  ``ManyToManyField`` from being recognized in migrations (:ticket:`24236`).