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

This reverts commit 1702bc52cc20ed0729893177fc8f4391b4b3183c.

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.

3 files changed, 77 insertions, 120 deletions

diff --git a/django/db/migrations/autodetector.py b/django/db/migrations/autodetector.py
index b0720dd51f..b323182df9 100644
--- a/django/db/migrations/autodetector.py
+++ b/django/db/migrations/autodetector.py
@@ -778,71 +778,73 @@ 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 = []
-        if field.rel and field.rel.to:
-            # Account for FKs to swappable models
-            swappable_setting = getattr(field, 'swappable_setting', None)
-            if swappable_setting is not None:
-                dep_app_label = "__setting__"
-                dep_object_name = swappable_setting
+            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 = []
+            if field.rel and field.rel.to:
+                # Account for FKs to swappable models
+                swappable_setting = getattr(field, 'swappable_setting', None)
+                if swappable_setting is not None:
+                    dep_app_label = "__setting__"
+                    dep_object_name = swappable_setting
+                else:
+                    dep_app_label = field.rel.to._meta.app_label
+                    dep_object_name = field.rel.to._meta.object_name
+                dependencies = [(dep_app_label, dep_object_name, None, True)]
+                if getattr(field.rel, "through", None) and not field.rel.through._meta.auto_created:
+                    dependencies.append((
+                        field.rel.through._meta.app_label,
+                        field.rel.through._meta.object_name,
+                        None,
+                        True
+                    ))
+            # You can't just add NOT NULL fields with no default or fields
+            # which don't allow empty strings as default.
+            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)
+                self.add_operation(
+                    app_label,
+                    operations.AddField(
+                        model_name=model_name,
+                        name=field_name,
+                        field=field,
+                        preserve_default=False,
+                    ),
+                    dependencies=dependencies,
+                )
             else:
-                dep_app_label = field.rel.to._meta.app_label
-                dep_object_name = field.rel.to._meta.object_name
-            dependencies = [(dep_app_label, dep_object_name, None, True)]
-            if getattr(field.rel, "through", None) and not field.rel.through._meta.auto_created:
-                dependencies.append((
-                    field.rel.through._meta.app_label,
-                    field.rel.through._meta.object_name,
-                    None,
-                    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,
-            ),
-            dependencies=dependencies,
-        )
+                self.add_operation(
+                    app_label,
+                    operations.AddField(
+                        model_name=model_name,
+                        name=field_name,
+                        field=field,
+                    ),
+                    dependencies=dependencies,
+                )
 
     def generate_removed_fields(self):
         """
         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(
-                model_name=model_name,
-                name=field_name,
-            ),
-            # We might need to depend on the removal of an
-            # order_with_respect_to or index/unique_together operation;
-            # this is safely ignored if there isn't one
-            dependencies=[
-                (app_label, model_name, field_name, "order_wrt_unset"),
-                (app_label, model_name, field_name, "foo_together_change"),
-            ],
-        )
+            self.add_operation(
+                app_label,
+                operations.RemoveField(
+                    model_name=model_name,
+                    name=field_name,
+                ),
+                # We might need to depend on the removal of an
+                # order_with_respect_to or index/unique_together operation;
+                # this is safely ignored if there isn't one
+                dependencies=[
+                    (app_label, model_name, field_name, "order_wrt_unset"),
+                    (app_label, model_name, field_name, "foo_together_change"),
+                ],
+            )
 
     def generate_altered_fields(self):
         """
@@ -866,30 +868,25 @@ 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
+                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)):
-                    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)):
-                        field = new_field.clone()
-                        new_default = self.questioner.ask_not_null_alteration(field_name, model_name)
-                        if new_default is not models.NOT_PROVIDED:
-                            field.default = new_default
-                            preserve_default = False
-                    else:
-                        field = new_field
-                    self.add_operation(
-                        app_label,
-                        operations.AlterField(
-                            model_name=model_name,
-                            name=field_name,
-                            field=field,
-                            preserve_default=preserve_default,
-                        )
-                    )
+                    field = new_field.clone()
+                    new_default = self.questioner.ask_not_null_alteration(field_name, model_name)
+                    if new_default is not models.NOT_PROVIDED:
+                        field.default = new_default
+                        preserve_default = False
                 else:
-                    self._generate_removed_field(app_label, model_name, field_name)
-                    self._generate_added_field(app_label, model_name, field_name)
+                    field = new_field
+                self.add_operation(
+                    app_label,
+                    operations.AlterField(
+                        model_name=model_name,
+                        name=field_name,
+                        field=field,
+                        preserve_default=preserve_default,
+                    )
+                )
 
     def _generate_altered_foo_together(self, operation):
         option_name = operation.option_name
diff --git a/docs/releases/1.7.2.txt b/docs/releases/1.7.2.txt
index 34f3eb4675..50cd0729f2 100644
--- a/docs/releases/1.7.2.txt
+++ b/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`).
diff --git a/tests/migrations/test_autodetector.py b/tests/migrations/test_autodetector.py
index d097d4f2da..8beda8e552 100644
--- a/tests/migrations/test_autodetector.py
+++ b/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