Revert "[1.7.x] Fixed #23938 -- Added migration support for m2m to concrete fields and vice versa"

This reverts commit 1702bc52cc.

This doesn't work on stable/1.7.x because #23844 wasn't backported and we're
not willing to do so because it's a large change.

django/db/migrations/autodetector.py

@@ -778,9 +778,6 @@ class MigrationAutodetector(object):
         Fields that have been added
         """
         for app_label, model_name, field_name in sorted(self.new_field_keys - self.old_field_keys):
-            self._generate_added_field(app_label, model_name, field_name)
-
-    def _generate_added_field(self, app_label, model_name, field_name):
             field = self.new_apps.get_model(app_label, model_name)._meta.get_field_by_name(field_name)[0]
             # Fields that are foreignkeys/m2ms depend on stuff
             dependencies = []
@@ -799,24 +796,32 @@ class MigrationAutodetector(object):
                         field.rel.through._meta.app_label,
                         field.rel.through._meta.object_name,
                         None,
-                    True,
+                        True
                     ))
             # You can't just add NOT NULL fields with no default or fields
             # which don't allow empty strings as default.
-        preserve_default = True
             if (not field.null and not field.has_default() and
                     not isinstance(field, models.ManyToManyField) and
                     not (field.blank and field.empty_strings_allowed)):
                 field = field.clone()
                 field.default = self.questioner.ask_not_null_addition(field_name, model_name)
-            preserve_default = False
                 self.add_operation(
                     app_label,
                     operations.AddField(
                         model_name=model_name,
                         name=field_name,
                         field=field,
-                preserve_default=preserve_default,
+                        preserve_default=False,
+                    ),
+                    dependencies=dependencies,
+                )
+            else:
+                self.add_operation(
+                    app_label,
+                    operations.AddField(
+                        model_name=model_name,
+                        name=field_name,
+                        field=field,
                     ),
                     dependencies=dependencies,
                 )
@@ -826,9 +831,6 @@ class MigrationAutodetector(object):
         Fields that have been removed.
         """
         for app_label, model_name, field_name in sorted(self.old_field_keys - self.new_field_keys):
-            self._generate_removed_field(app_label, model_name, field_name)
-
-    def _generate_removed_field(self, app_label, model_name, field_name):
             self.add_operation(
                 app_label,
                 operations.RemoveField(
@@ -866,8 +868,6 @@ class MigrationAutodetector(object):
             old_field_dec = self.deep_deconstruct(old_field)
             new_field_dec = self.deep_deconstruct(new_field)
             if old_field_dec != new_field_dec:
-                if (not isinstance(old_field, models.ManyToManyField) and
-                        not isinstance(new_field, models.ManyToManyField)):
                 preserve_default = True
                 if (old_field.null and not new_field.null and not new_field.has_default() and
                         not isinstance(new_field, models.ManyToManyField)):
@@ -887,9 +887,6 @@ class MigrationAutodetector(object):
                         preserve_default=preserve_default,
                     )
                 )
-                else:
-                    self._generate_removed_field(app_label, model_name, field_name)
-                    self._generate_added_field(app_label, model_name, field_name)
 
     def _generate_altered_foo_together(self, operation):
         option_name = operation.option_name

docs/releases/1.7.2.txt

@@ -181,6 +181,3 @@ Bugfixes
 
 * Supported strings escaped by third-party libraries with the ``__html__``
   convention in the template engine (:ticket:`23831`).
-
-* Fixed a migration crash when changing a ``ManyToManyField`` into a concrete
-  field and vice versa (:ticket:`23938`).

tests/migrations/test_autodetector.py

@@ -116,10 +116,6 @@ class AutodetectorTests(TestCase):
         ("id", models.AutoField(primary_key=True)),
         ("publishers", models.ManyToManyField("testapp.Publisher", through="testapp.Contract")),
     ])
-    author_with_former_m2m = ModelState("testapp", "Author", [
-        ("id", models.AutoField(primary_key=True)),
-        ("publishers", models.CharField(max_length=100)),
-    ])
     author_with_options = ModelState("testapp", "Author", [
         ("id", models.AutoField(primary_key=True)),
     ], {
@@ -1278,39 +1274,6 @@ class AutodetectorTests(TestCase):
         self.assertOperationAttributes(changes, "testapp", 0, 3, name="publisher", model_name='contract')
         self.assertOperationAttributes(changes, "testapp", 0, 4, name="Contract")
 
-    def test_concrete_field_changed_to_many_to_many(self):
-        """
-        #23938 - Tests that changing a concrete field into a ManyToManyField
-        first removes the concrete field and then adds the m2m field.
-        """
-        before = self.make_project_state([self.author_with_former_m2m])
-        after = self.make_project_state([self.author_with_m2m, self.publisher])
-        autodetector = MigrationAutodetector(before, after)
-        changes = autodetector._detect_changes()
-        # Right number/type of migrations?
-        self.assertNumberMigrations(changes, "testapp", 1)
-        self.assertOperationTypes(changes, "testapp", 0, ["CreateModel", "RemoveField", "AddField"])
-        self.assertOperationAttributes(changes, 'testapp', 0, 0, name='Publisher')
-        self.assertOperationAttributes(changes, 'testapp', 0, 1, name="publishers", model_name='author')
-        self.assertOperationAttributes(changes, 'testapp', 0, 2, name="publishers", model_name='author')
-
-    def test_many_to_many_changed_to_concrete_field(self):
-        """
-        #23938 - Tests that changing a ManyToManyField into a concrete field
-        first removes the m2m field and then adds the concrete field.
-        """
-        before = self.make_project_state([self.author_with_m2m, self.publisher])
-        after = self.make_project_state([self.author_with_former_m2m])
-        autodetector = MigrationAutodetector(before, after)
-        changes = autodetector._detect_changes()
-        # Right number/type of migrations?
-        self.assertNumberMigrations(changes, "testapp", 1)
-        self.assertOperationTypes(changes, "testapp", 0, ["RemoveField", "AddField", "DeleteModel"])
-        self.assertOperationAttributes(changes, 'testapp', 0, 0, name="publishers", model_name='author')
-        self.assertOperationAttributes(changes, 'testapp', 0, 1, name="publishers", model_name='author')
-        self.assertOperationAttributes(changes, 'testapp', 0, 2, name='Publisher')
-        self.assertOperationFieldAttributes(changes, 'testapp', 0, 1, max_length=100)
-
     def test_non_circular_foreignkey_dependency_removal(self):
         """
         If two models with a ForeignKey from one to the other are removed at the