Fixed #23071 -- Use last migration's name in dependency to other app

Changed the autodetector to lookup the name of the other app's last
migration in the graph and use that as dependency instead of using
__latest__.

django/db/migrations/autodetector.py

@@ -254,13 +254,13 @@ class MigrationAutodetector(object):
                                     # If we can't find the other app, we add a first/last dependency,
                                     # but only if we've already been through once and checked everything
                                     if chop_mode:
-                                        # If the app already exists, we add __latest__, as we don't know which
+                                        # If the app already exists, we add a dependency on the last migration,
-                                        # migration contains the target field.
+                                        # as we don't know which migration contains the target field.
                                         # If it's not yet migrated or has no migrations, we use __first__
-                                        if graph and not graph.root_nodes(dep[0]):
+                                        if graph and graph.leaf_nodes(dep[0]):
-                                            operation_dependencies.add((dep[0], "__first__"))
+                                            operation_dependencies.add(graph.leaf_nodes(dep[0])[0])
                                         else:
-                                            operation_dependencies.add((dep[0], "__latest__"))
+                                            operation_dependencies.add((dep[0], "__first__"))
                                     else:
                                         deps_satisfied = False
                     if deps_satisfied:

django/db/migrations/loader.py

@@ -136,7 +136,7 @@ class MigrationLoader(object):
             return self.disk_migrations[results[0]]
 
     def check_key(self, key, current_app):
-        if (key[1] != "__first__" and key[1] != "__latest__") or key in self.graph:
+        if key[1] != "__first__" or key in self.graph:
             return key
         # Special-case __first__, which means "the first migration" for
         # migrated apps, and is ignored for unmigrated apps. It allows

tests/migrations/test_autodetector.py

@@ -1059,7 +1059,8 @@ class AutodetectorTests(TestCase):
     @override_settings(MIGRATION_MODULES={"migrations": "migrations.test_migrations"})
     def test_last_dependency(self):
         """
-        Tests that a dependency to an app with existing migrations uses __latest__.
+        Tests that a dependency to an app with existing migrations uses the
+        last migration of that app.
         """
         # Load graph
         loader = MigrationLoader(connection)
@@ -1074,4 +1075,4 @@ class AutodetectorTests(TestCase):
         self.assertOperationTypes(changes, 'otherapp', 0, ["CreateModel"])
         self.assertOperationAttributes(changes, 'otherapp', 0, 0, name="Book")
         # Right dependencies?
-        self.assertEqual(changes['otherapp'][0].dependencies, [("migrations", "__latest__")])
+        self.assertEqual(changes['otherapp'][0].dependencies, [("migrations", "0002_second")])

tests/migrations/test_loader.py

@@ -122,26 +122,6 @@ class LoaderTests(TestCase):
             ],
         )
 
-    @modify_settings(INSTALLED_APPS={'append': 'basic'})
-    @override_settings(MIGRATION_MODULES={
-        "migrations": "migrations.test_migrations_latest",
-        "basic": "migrations.test_migrations_latest_basic",
-    })
-    def test_latest(self):
-        """
-        Makes sure that __latest__ works correctly.
-        """
-        # Load and test the plan
-        migration_loader = MigrationLoader(connection)
-        self.assertEqual(
-            migration_loader.graph.forwards_plan(("migrations", "0001_initial")),
-            [
-                ("basic", "0001_initial"),
-                ("basic", "0002_second"),
-                ("migrations", "0001_initial"),
-            ],
-        )
-
     @override_settings(MIGRATION_MODULES={
         "migrations": "migrations.test_migrations_first",
         "migrations2": "migrations2.test_migrations_2_first",

tests/migrations/test_migrations_latest/0001_initial.py

@@ -1,13 +0,0 @@
-# -*- coding: utf-8 -*-
-from __future__ import unicode_literals
-
-from django.db import migrations
-
-
-class Migration(migrations.Migration):
-
-    dependencies = [
-        ("basic", "__latest__"),
-    ]
-
-    operations = []

tests/migrations/test_migrations_latest_basic/0001_initial.py

@@ -1,11 +0,0 @@
-# -*- coding: utf-8 -*-
-from __future__ import unicode_literals
-
-from django.db import migrations
-
-
-class Migration(migrations.Migration):
-
-    dependencies = []
-
-    operations = []

tests/migrations/test_migrations_latest_basic/0002_second.py

@@ -1,13 +0,0 @@
-# -*- coding: utf-8 -*-
-from __future__ import unicode_literals
-
-from django.db import migrations
-
-
-class Migration(migrations.Migration):
-
-    dependencies = [
-        ("basic", "0001_initial"),
-    ]
-
-    operations = []