[1.7.x] Revert "Fixed #23474 -- Prevented migrating backwards from unapplying the wrong migrations."

Backport of d7ab2cefb7 from master

django/db/migrations/executor.py

@@ -36,15 +36,11 @@ class MigrationExecutor(object):
             # If the migration is already applied, do backwards mode,
             # otherwise do forwards mode.
             elif target in applied:
-                app_label = target[0]
-                next_migration_prefix = str(int(target[1][:4]) + 1)
-                try:
-                    next_migration = self.loader.get_migration_by_prefix(app_label, next_migration_prefix)
-                except KeyError:
-                    # If `target` is the most recent one in its app, there is nothing to do.
-                    pass
-                else:
-                    backwards_plan = self.loader.graph.backwards_plan(next_migration)
+                backwards_plan = self.loader.graph.backwards_plan(target)[:-1]
+                # We only do this if the migration is not the most recent one
+                # in its app - that is, another migration with the same app
+                # label is in the backwards plan
+                if any(node[0] == target[0] for node in backwards_plan):
                     for migration in backwards_plan:
                         if migration in applied:
                             plan.append((self.loader.graph.nodes[migration], True))

docs/releases/1.7.1.txt

@@ -47,9 +47,6 @@ Bugfixes
 * Allowed migrations to work with ``app_label``\s that have the same last
   part (e.g. ``django.contrib.auth`` and ``vendor.auth``) (:ticket:`23483`).
 
-* Fixed bug in migrations that could cause unexpected data loss when executing
-  a backwards or no-op migration (:ticket:`23474`).
-
 * Restored the ability to deepcopy ``F`` objects (:ticket:`23492`).
 
 * Formats for Welsh (``cy``) and several Chinese locales (``zh_CN``,

tests/migrations/test_executor.py

@@ -231,41 +231,3 @@ class ExecutorTests(MigrationTestBase):
         executor.migrate([("migrations", None)])
         self.assertTableNotExists("migrations_author")
         self.assertTableNotExists("migrations_tribble")
-
-    @override_settings(
-        MIGRATION_MODULES={
-            "migrations": "migrations.test_migrations_backwards_deps_1",
-            "migrations2": "migrations2.test_migrations_backwards_deps_2",
-        },
-    )
-    def test_backwards_deps(self):
-        """
-        #23474 - Migrating backwards shouldn't cause the wrong migrations to be
-        unapplied.
-
-        Migration dependencies (x -> y === y depends on x):
-        m.0001 -+-> m.0002
-                +-> m2.0001
-
-        1) Migrate m2 to 0001, causing { m.0001, m2.0002 } to be applied.
-        2) Migrate m to 0001. m.0001 has already been applied, so this should
-           be a noop.
-        """
-        executor = MigrationExecutor(connection)
-        executor.migrate([("migrations2", "0001_initial")])
-        try:
-            self.assertTableExists("migrations2_example")
-            # Rebuild the graph to reflect the new DB state
-            executor.loader.build_graph()
-            self.assertEqual(
-                executor.migration_plan([("migrations", "0001_initial")]),
-                [],
-            )
-            executor.migrate([("migrations", "0001_initial")])
-            self.assertTableExists("migrations2_example")
-        finally:
-            # And migrate back to clean up the database
-            executor.loader.build_graph()
-            executor.migrate([("migrations", None)])
-            self.assertTableNotExists("migrations_author")
-            self.assertTableNotExists("migrations_tribble")

tests/migrations2/test_migrations_backwards_deps_2/0001_initial.py

@@ -1,16 +0,0 @@
-# -*- coding: utf-8 -*-
-from __future__ import unicode_literals
-
-from django.db import migrations, models
-
-
-class Migration(migrations.Migration):
-    dependencies = [('migrations', '0001_initial')]
-    operations = [
-        migrations.CreateModel(
-            "Example",
-            [
-                ("id", models.AutoField(primary_key=True)),
-            ],
-        ),
-    ]