[1.7.x] Fixed #24104 -- Fixed check to look on field.get_internal_type() instead of class instance

5 files changed, 109 insertions, 8 deletions

diff --git a/django/db/backends/schema.py b/django/db/backends/schema.py
index 7c10db7dec..12e2ab7657 100644
--- a/django/db/backends/schema.py
+++ b/django/db/backends/schema.py
@@ -3,7 +3,6 @@ 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
@@ -359,7 +358,7 @@ class BaseDatabaseSchemaEditor(object):
         table instead (for M2M fields)
         """
         # Special-case implicit M2M tables
-        if isinstance(field, ManyToManyField) and field.rel.through._meta.auto_created:
+        if 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)
@@ -403,7 +402,7 @@ class BaseDatabaseSchemaEditor(object):
         but for M2Ms may involve deleting a table.
         """
         # Special-case implicit M2M tables
-        if isinstance(field, ManyToManyField) and field.rel.through._meta.auto_created:
+        if 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 73c70effb9..c5f56a04a7 100644
--- a/django/db/backends/sqlite3/schema.py
+++ b/django/db/backends/sqlite3/schema.py
@@ -4,7 +4,6 @@ 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):
@@ -70,7 +69,7 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         for field in create_fields:
             body[field.name] = field
             # Choose a default and insert it into the copy map
-            if not isinstance(field, ManyToManyField):
+            if not field.get_internal_type() == 'ManyToManyField':
                 mapping[field.column] = self.quote_value(
                     self.effective_default(field)
                 )
@@ -93,7 +92,7 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
             del body[field.name]
             del mapping[field.column]
             # Remove any implicit M2M tables
-            if isinstance(field, ManyToManyField) and field.rel.through._meta.auto_created:
+            if 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()
@@ -172,7 +171,7 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         table instead (for M2M fields)
         """
         # Special-case implicit M2M tables
-        if isinstance(field, ManyToManyField) and field.rel.through._meta.auto_created:
+        if 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])
 
@@ -182,7 +181,7 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         but for M2Ms may involve deleting a table.
         """
         # M2M fields are a special case
-        if isinstance(field, ManyToManyField):
+        if 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/1.7.4.txt b/docs/releases/1.7.4.txt
index 0d53459068..133dd44369 100644
--- a/docs/releases/1.7.4.txt
+++ b/docs/releases/1.7.4.txt
@@ -20,3 +20,7 @@ Bugfixes
 
 * Prevented the ``static.serve`` view from producing ``ResourceWarning``\s in
   certain circumstances (security fix regression, :ticket:`24193`).
+
+* Fixed schema check for ``ManyToManyField`` to look for internal type instead
+  of checking class instance, so you can write custom m2m-like fields with the
+  same behavior. (:ticket:`24104`).
diff --git a/tests/schema/fields.py b/tests/schema/fields.py
new file mode 100644
index 0000000000..596671a6bc
--- /dev/null
+++ b/tests/schema/fields.py
@@ -0,0 +1,54 @@
+from django.db.models.fields.related import (
+    create_many_to_many_intermediary_model,
+    ManyToManyField, ManyToManyRel, RelatedField,
+    RECURSIVE_RELATIONSHIP_CONSTANT, ReverseManyRelatedObjectsDescriptor,
+)
+from django.utils.functional import curry
+
+
+class CustomManyToManyField(RelatedField):
+    """
+    Ticket #24104 - Need to have a custom ManyToManyField,
+    which is not an inheritor of ManyToManyField.
+    """
+
+    def __init__(self, to, db_constraint=True, swappable=True, **kwargs):
+        try:
+            to._meta
+        except AttributeError:
+            to = str(to)
+        kwargs['verbose_name'] = kwargs.get('verbose_name', None)
+        kwargs['rel'] = ManyToManyRel(
+            to,
+            related_name=kwargs.pop('related_name', None),
+            related_query_name=kwargs.pop('related_query_name', None),
+            limit_choices_to=kwargs.pop('limit_choices_to', None),
+            symmetrical=kwargs.pop('symmetrical', to == RECURSIVE_RELATIONSHIP_CONSTANT),
+            through=kwargs.pop('through', None),
+            through_fields=kwargs.pop('through_fields', None),
+            db_constraint=db_constraint,
+        )
+        self.swappable = swappable
+        self.db_table = kwargs.pop('db_table', None)
+        if kwargs['rel'].through is not None:
+            assert self.db_table is None, "Cannot specify a db_table if an intermediary model is used."
+        super(CustomManyToManyField, self).__init__(**kwargs)
+
+    def contribute_to_class(self, cls, name):
+        if self.rel.symmetrical and (self.rel.to == "self" or self.rel.to == cls._meta.object_name):
+            self.rel.related_name = "%s_rel_+" % name
+        super(CustomManyToManyField, self).contribute_to_class(cls, name)
+        if not self.rel.through and not cls._meta.abstract and not cls._meta.swapped:
+            self.rel.through = create_many_to_many_intermediary_model(self, cls)
+        setattr(cls, self.name, ReverseManyRelatedObjectsDescriptor(self))
+        self.m2m_db_table = curry(self._get_m2m_db_table, cls._meta)
+
+    def get_internal_type(self):
+        return 'ManyToManyField'
+
+    # Copy those methods from ManyToManyField because they don't call super() internally
+    contribute_to_related_class = ManyToManyField.__dict__['contribute_to_related_class']
+    set_attributes_from_rel = ManyToManyField.__dict__['set_attributes_from_rel']
+    _get_m2m_attr = ManyToManyField.__dict__['_get_m2m_attr']
+    _get_m2m_reverse_attr = ManyToManyField.__dict__['_get_m2m_reverse_attr']
+    _get_m2m_db_table = ManyToManyField.__dict__['_get_m2m_db_table']
diff --git a/tests/schema/tests.py b/tests/schema/tests.py
index f86916b5a4..a0ca9f19eb 100644
--- a/tests/schema/tests.py
+++ b/tests/schema/tests.py
@@ -7,6 +7,7 @@ from django.db.models.fields import (BinaryField, BooleanField, CharField, Integ
     PositiveIntegerField, SlugField, TextField)
 from django.db.models.fields.related import ForeignKey, ManyToManyField, OneToOneField
 from django.db.transaction import atomic
+from .fields import CustomManyToManyField
 from .models import (Author, AuthorWithDefaultHeight, AuthorWithM2M, Book, BookWithLongName,
     BookWithSlug, BookWithM2M, Tag, TagIndexed, TagM2MTest, TagUniqueRename,
     UniqueTest, Thing, TagThrough, BookWithM2MThrough, AuthorTag, AuthorWithM2MThrough,
@@ -1310,3 +1311,47 @@ class SchemaTests(TransactionTestCase):
             cursor.execute("SELECT surname FROM schema_author;")
             item = cursor.fetchall()[0]
             self.assertEqual(item[0], None if connection.features.interprets_empty_strings_as_nulls else '')
+
+    def test_custom_manytomanyfield(self):
+        """
+        #24104 - Schema editors should look for internal type of field
+        """
+        # Create the tables
+        with connection.schema_editor() as editor:
+            editor.create_model(AuthorWithM2M)
+            editor.create_model(TagM2MTest)
+        # Create an M2M field
+        new_field = CustomManyToManyField("schema.TagM2MTest", related_name="authors")
+        new_field.contribute_to_class(AuthorWithM2M, "tags")
+        # Ensure there's no m2m table there
+        self.assertRaises(DatabaseError, self.column_classes, new_field.rel.through)
+        try:
+            # Add the field
+            with connection.schema_editor() as editor:
+                editor.add_field(
+                    AuthorWithM2M,
+                    new_field,
+                )
+            # Ensure there is now an m2m table there
+            columns = self.column_classes(new_field.rel.through)
+            self.assertEqual(columns['tagm2mtest_id'][0], "IntegerField")
+
+            # "Alter" the field. This should not rename the DB table to itself.
+            with connection.schema_editor() as editor:
+                editor.alter_field(
+                    AuthorWithM2M,
+                    new_field,
+                    new_field,
+                )
+
+            # Remove the M2M table again
+            with connection.schema_editor() as editor:
+                editor.remove_field(
+                    AuthorWithM2M,
+                    new_field,
+                )
+            # Ensure there's no m2m table there
+            self.assertRaises(DatabaseError, self.column_classes, new_field.rel.through)
+        finally:
+            # Cleanup model states
+            AuthorWithM2M._meta.local_many_to_many.remove(new_field)