Add M2M repointing

4 files changed, 119 insertions, 16 deletions

diff --git a/django/db/backends/schema.py b/django/db/backends/schema.py
index 55d687a7f6..bd86d52e88 100644
--- a/django/db/backends/schema.py
+++ b/django/db/backends/schema.py
@@ -21,7 +21,6 @@ class BaseDatabaseSchemaEditor(object):
     commit() is called.
 
     TODO:
-        - Repointing of M2Ms
         - Check constraints (PosIntField)
     """
 
@@ -154,13 +153,13 @@ class BaseDatabaseSchemaEditor(object):
 
     # Actions
 
-    def create_model(self, model):
+    def create_model(self, model, force=False):
         """
         Takes a model and creates a table for it in the database.
         Will also create any accompanying indexes or unique constraints.
         """
         # Do nothing if this is an unmanaged or proxy model
-        if not model._meta.managed or model._meta.proxy:
+        if not force and (not model._meta.managed or model._meta.proxy):
             return
         # Create column SQL, add FK deferreds if needed
         column_sqls = []
@@ -214,13 +213,16 @@ class BaseDatabaseSchemaEditor(object):
             "definition": ", ".join(column_sqls)
         }
         self.execute(sql, params)
+        # Make M2M tables
+        for field in model._meta.local_many_to_many:
+            self.create_model(field.rel.through, force=True)
 
-    def delete_model(self, model):
+    def delete_model(self, model, force=False):
         """
         Deletes a model from the database.
         """
         # Do nothing if this is an unmanaged or proxy model
-        if not model._meta.managed or model._meta.proxy:
+        if not force and (not model._meta.managed or model._meta.proxy):
             return
         # Delete the table
         self.execute(self.sql_delete_table % {
@@ -287,7 +289,7 @@ class BaseDatabaseSchemaEditor(object):
         """
         # Special-case implicit M2M tables
         if isinstance(field, ManyToManyField) and field.rel.through._meta.auto_created:
-            return self.create_model(field.rel.through)
+            return self.create_model(field.rel.through, force=True)
         # Get the column's definition
         definition, params = self.column_sql(model, field, include_default=True)
         # It might not actually have a column behind it
@@ -358,11 +360,10 @@ class BaseDatabaseSchemaEditor(object):
         # Ensure this field is even column-based
         old_type = old_field.db_type(connection=self.connection)
         new_type = self._type_for_alter(new_field)
-        if old_type is None and new_type is None:
-            # TODO: Handle M2M fields being repointed
-            return
+        if old_type is None and new_type is None and (old_field.rel.through and new_field.rel.through and old_field.rel.through._meta.auto_created and new_field.rel.through._meta.auto_created):
+            return self._alter_many_to_many(model, old_field, new_field, strict)
         elif old_type is None or new_type is None:
-            raise ValueError("Cannot alter field %s into %s - they are not compatible types" % (
+            raise ValueError("Cannot alter field %s into %s - they are not compatible types (probably means only one is an M2M with implicit through model)" % (
                     old_field,
                     new_field,
                 ))
@@ -543,6 +544,17 @@ class BaseDatabaseSchemaEditor(object):
                 }
             )
 
+    def _alter_many_to_many(self, model, old_field, new_field, strict):
+        "Alters M2Ms to repoint their to= endpoints."
+        # Rename the through table
+        self.alter_db_table(old_field.rel.through, old_field.rel.through._meta.db_table, new_field.rel.through._meta.db_table)
+        # Repoint the FK to the other side
+        self.alter_field(
+            new_field.rel.through,
+            old_field.rel.through._meta.get_field_by_name(old_field.m2m_reverse_field_name())[0],
+            new_field.rel.through._meta.get_field_by_name(new_field.m2m_reverse_field_name())[0],
+        )
+
     def _type_for_alter(self, field):
         """
         Returns a field's type suitable for ALTER COLUMN.
diff --git a/django/db/backends/sqlite3/schema.py b/django/db/backends/sqlite3/schema.py
index 7938ad79cf..e660f26c87 100644
--- a/django/db/backends/sqlite3/schema.py
+++ b/django/db/backends/sqlite3/schema.py
@@ -101,11 +101,10 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         # Ensure this field is even column-based
         old_type = old_field.db_type(connection=self.connection)
         new_type = self._type_for_alter(new_field)
-        if old_type is None and new_type is None:
-            # TODO: Handle M2M fields being repointed
-            return
+        if old_type is None and new_type is None and (old_field.rel.through and new_field.rel.through and old_field.rel.through._meta.auto_created and new_field.rel.through._meta.auto_created):
+            return self._alter_many_to_many(model, old_field, new_field, strict)
         elif old_type is None or new_type is None:
-            raise ValueError("Cannot alter field %s into %s - they are not compatible types" % (
+            raise ValueError("Cannot alter field %s into %s - they are not compatible types (probably means only one is an M2M with implicit through model)" % (
                     old_field,
                     new_field,
                 ))
@@ -114,3 +113,25 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
 
     def alter_unique_together(self, model, old_unique_together, new_unique_together):
         self._remake_table(model, override_uniques=new_unique_together)
+
+    def _alter_many_to_many(self, model, old_field, new_field, strict):
+        "Alters M2Ms to repoint their to= endpoints."
+        # Make a new through table
+        self.create_model(new_field.rel.through)
+        # Copy the data across
+        self.execute("INSERT INTO %s (%s) SELECT %s FROM %s;" % (
+            self.quote_name(new_field.rel.through._meta.db_table),
+            ', '.join([
+                "id",
+                new_field.m2m_column_name(),
+                new_field.m2m_reverse_name(),
+            ]),
+            ', '.join([
+                "id",
+                old_field.m2m_column_name(),
+                old_field.m2m_reverse_name(),
+            ]),
+            self.quote_name(old_field.rel.through._meta.db_table),
+        ))
+        # Delete the old through table
+        self.delete_model(old_field.rel.through, force=True)
diff --git a/tests/modeltests/schema/models.py b/tests/modeltests/schema/models.py
index b18d2a9c16..76a8cf3687 100644
--- a/tests/modeltests/schema/models.py
+++ b/tests/modeltests/schema/models.py
@@ -29,6 +29,16 @@ class Book(models.Model):
         managed = False
 
 
+class BookWithM2M(models.Model):
+    author = models.ForeignKey(Author)
+    title = models.CharField(max_length=100, db_index=True)
+    pub_date = models.DateTimeField()
+    tags = models.ManyToManyField("Tag", related_name="books")
+
+    class Meta:
+        managed = False
+
+
 class BookWithSlug(models.Model):
     author = models.ForeignKey(Author)
     title = models.CharField(max_length=100, db_index=True)
diff --git a/tests/modeltests/schema/tests.py b/tests/modeltests/schema/tests.py
index 310f74a4e1..b3fc5d1c80 100644
--- a/tests/modeltests/schema/tests.py
+++ b/tests/modeltests/schema/tests.py
@@ -7,7 +7,7 @@ from django.db import connection, DatabaseError, IntegrityError
 from django.db.models.fields import IntegerField, TextField, CharField, SlugField
 from django.db.models.fields.related import ManyToManyField, ForeignKey
 from django.db.models.loading import cache
-from .models import Author, Book, BookWithSlug, AuthorWithM2M, Tag, TagUniqueRename, UniqueTest
+from .models import Author, Book, BookWithSlug, BookWithM2M, AuthorWithM2M, Tag, TagUniqueRename, UniqueTest
 
 
 class SchemaTests(TestCase):
@@ -19,7 +19,7 @@ class SchemaTests(TestCase):
     as the code it is testing.
     """
 
-    models = [Author, Book, BookWithSlug, AuthorWithM2M, Tag, TagUniqueRename, UniqueTest]
+    models = [Author, Book, BookWithSlug, BookWithM2M, AuthorWithM2M, Tag, TagUniqueRename, UniqueTest]
 
     # Utility functions
 
@@ -248,6 +248,21 @@ class SchemaTests(TestCase):
         self.assertEqual(columns['display_name'][0], "CharField")
         self.assertNotIn("name", columns)
 
+    def test_m2m_create(self):
+        """
+        Tests M2M fields on models during creation
+        """
+        # Create the tables
+        editor = connection.schema_editor()
+        editor.start()
+        editor.create_model(Author)
+        editor.create_model(Tag)
+        editor.create_model(BookWithM2M)
+        editor.commit()
+        # Ensure there is now an m2m table there
+        columns = self.column_classes(BookWithM2M._meta.get_field_by_name("tags")[0].rel.through)
+        self.assertEqual(columns['tag_id'][0], "IntegerField")
+
     def test_m2m(self):
         """
         Tests adding/removing M2M fields on models
@@ -287,6 +302,51 @@ class SchemaTests(TestCase):
         self.assertRaises(DatabaseError, self.column_classes, new_field.rel.through)
         connection.rollback()
 
+    def test_m2m_repoint(self):
+        """
+        Tests repointing M2M fields
+        """
+        # Create the tables
+        editor = connection.schema_editor()
+        editor.start()
+        editor.create_model(Author)
+        editor.create_model(BookWithM2M)
+        editor.create_model(Tag)
+        editor.create_model(UniqueTest)
+        editor.commit()
+        # Ensure the M2M exists and points to Tag
+        constraints = connection.introspection.get_constraints(connection.cursor(), BookWithM2M._meta.get_field_by_name("tags")[0].rel.through._meta.db_table)
+        if connection.features.supports_foreign_keys:
+            for name, details in constraints.items():
+                if details['columns'] == set(["tag_id"]) and details['foreign_key']:
+                    self.assertEqual(details['foreign_key'], ('schema_tag', 'id'))
+                    break
+            else:
+                self.fail("No FK constraint for tag_id found")
+        # Repoint the M2M
+        new_field = ManyToManyField(UniqueTest)
+        new_field.contribute_to_class(BookWithM2M, "uniques")
+        editor = connection.schema_editor()
+        editor.start()
+        editor.alter_field(
+            Author,
+            BookWithM2M._meta.get_field_by_name("tags")[0],
+            new_field,
+        )
+        editor.commit()
+        # Ensure old M2M is gone
+        self.assertRaises(DatabaseError, self.column_classes, BookWithM2M._meta.get_field_by_name("tags")[0].rel.through)
+        connection.rollback()
+        # Ensure the new M2M exists and points to UniqueTest
+        constraints = connection.introspection.get_constraints(connection.cursor(), new_field.rel.through._meta.db_table)
+        if connection.features.supports_foreign_keys:
+            for name, details in constraints.items():
+                if details['columns'] == set(["uniquetest_id"]) and details['foreign_key']:
+                    self.assertEqual(details['foreign_key'], ('schema_uniquetest', 'id'))
+                    break
+            else:
+                self.fail("No FK constraint for tag_id found")
+
     def test_unique(self):
         """
         Tests removing and adding unique constraints to a single column.