Fixed #24828 -- Allowed migration optimization across AlterFooTogether

The idea behind this change is, that AlterUniqueTogether,
AlterIndexTogether and AlterOrderWithRespectTo can always be moved after
an Add/Alter/Rename/RemoveField operation if they don't refer to the
respective field and are not empty sets / None.

Combined with the optimizations of duplicate AlterUniqueTogether,
AlterIndexTogether, and AlterOrderWithRespectTo operations from
128caa1e16ec2627737748f75c8e55600a3df97f, these operations are optimized
in a later round of the optimizer.

Thanks Tim Graham for the review.

5 files changed, 208 insertions, 4 deletions

diff --git a/django/db/migrations/operations/models.py b/django/db/migrations/operations/models.py
index 69998331fb..2855107e0e 100644
--- a/django/db/migrations/operations/models.py
+++ b/django/db/migrations/operations/models.py
@@ -365,6 +365,15 @@ class AlterUniqueTogether(Operation):
     def references_model(self, name, app_label=None):
         return name.lower() == self.name_lower
 
+    def references_field(self, model_name, name, app_label=None):
+        return (
+            self.references_model(model_name, app_label) and
+            (
+                not self.unique_together or
+                any((name in together) for together in self.unique_together)
+            )
+        )
+
     def describe(self):
         return "Alter %s for %s (%s constraint(s))" % (self.option_name, self.name, len(self.unique_together or ''))
 
@@ -417,6 +426,15 @@ class AlterIndexTogether(Operation):
     def references_model(self, name, app_label=None):
         return name.lower() == self.name_lower
 
+    def references_field(self, model_name, name, app_label=None):
+        return (
+            self.references_model(model_name, app_label) and
+            (
+                not self.index_together or
+                any((name in together) for together in self.index_together)
+            )
+        )
+
     def describe(self):
         return "Alter %s for %s (%s constraint(s))" % (self.option_name, self.name, len(self.index_together or ''))
 
@@ -474,6 +492,15 @@ class AlterOrderWithRespectTo(Operation):
     def references_model(self, name, app_label=None):
         return name.lower() == self.name_lower
 
+    def references_field(self, model_name, name, app_label=None):
+        return (
+            self.references_model(model_name, app_label) and
+            (
+                self.order_with_respect_to is None or
+                name == self.order_with_respect_to
+            )
+        )
+
     def describe(self):
         return "Set order_with_respect_to on %s to %s" % (self.name, self.order_with_respect_to)
 
diff --git a/django/db/migrations/optimizer.py b/django/db/migrations/optimizer.py
index 556436b96d..96909e748e 100644
--- a/django/db/migrations/optimizer.py
+++ b/django/db/migrations/optimizer.py
@@ -56,6 +56,20 @@ class MigrationOptimizer(object):
             (AlterField, RenameField): self.reduce_alter_field_rename_field,
             (CreateModel, RenameField): self.reduce_create_model_rename_field,
             (RenameField, RenameField): self.reduce_rename_field_self,
+
+            (AlterIndexTogether, AddField): self.reduce_alter_model_addalterremove_field,
+            (AlterIndexTogether, AlterField): self.reduce_alter_model_addalterremove_field,
+            (AlterIndexTogether, RemoveField): self.reduce_alter_model_addalterremove_field,
+            (AlterOrderWithRespectTo, AddField): self.reduce_alter_model_addalterremove_field,
+            (AlterOrderWithRespectTo, AlterField): self.reduce_alter_model_addalterremove_field,
+            (AlterOrderWithRespectTo, RemoveField): self.reduce_alter_model_addalterremove_field,
+            (AlterUniqueTogether, AddField): self.reduce_alter_model_addalterremove_field,
+            (AlterUniqueTogether, AlterField): self.reduce_alter_model_addalterremove_field,
+            (AlterUniqueTogether, RemoveField): self.reduce_alter_model_addalterremove_field,
+
+            (AlterIndexTogether, RenameField): self.reduce_alter_model_rename_field,
+            (AlterOrderWithRespectTo, RenameField): self.reduce_alter_model_rename_field,
+            (AlterUniqueTogether, RenameField): self.reduce_alter_model_rename_field,
         }
 
     def optimize(self, operations, app_label=None):
@@ -310,6 +324,16 @@ class MigrationOptimizer(object):
                 ),
             ]
 
+    def reduce_alter_model_addalterremove_field(self, operation, other, in_between):
+        if (operation.name_lower == other.model_name_lower and
+                not operation.references_field(other.model_name, other.name)):
+            return [other, operation]
+
+    def reduce_alter_model_rename_field(self, operation, other, in_between):
+        if (operation.name_lower == other.model_name_lower and
+                not operation.references_field(other.model_name, other.old_name)):
+            return [other, operation]
+
     # THROUGH CHECKS
 
     def can_optimize_through(self, operation, other, app_label=None):
diff --git a/tests/migrations/test_autodetector.py b/tests/migrations/test_autodetector.py
index d5ddd30894..e6497314dd 100644
--- a/tests/migrations/test_autodetector.py
+++ b/tests/migrations/test_autodetector.py
@@ -1149,9 +1149,10 @@ class AutodetectorTests(TestCase):
         changes = autodetector._detect_changes()
         # Right number/type of migrations?
         self.assertNumberMigrations(changes, "otherapp", 1)
-        self.assertOperationTypes(changes, "otherapp", 0, ["AlterUniqueTogether", "AlterIndexTogether", "RemoveField"])
-        self.assertOperationAttributes(changes, "otherapp", 0, 0, name="book", unique_together={("author", "title")})
-        self.assertOperationAttributes(changes, "otherapp", 0, 1, name="book", index_together={("author", "title")})
+        self.assertOperationTypes(changes, "otherapp", 0, ["RemoveField", "AlterUniqueTogether", "AlterIndexTogether"])
+        self.assertOperationAttributes(changes, "otherapp", 0, 0, model_name="book", name="newfield")
+        self.assertOperationAttributes(changes, "otherapp", 0, 1, name="book", unique_together={("author", "title")})
+        self.assertOperationAttributes(changes, "otherapp", 0, 2, name="book", index_together={("author", "title")})
 
     def test_rename_field_and_foo_together(self):
         """
diff --git a/tests/migrations/test_commands.py b/tests/migrations/test_commands.py
index ac4f55fa03..3077dd9e61 100644
--- a/tests/migrations/test_commands.py
+++ b/tests/migrations/test_commands.py
@@ -889,7 +889,7 @@ class SquashMigrationsTests(MigrationTestBase):
         out = six.StringIO()
         with self.temporary_migration_module(module="migrations.test_migrations"):
             call_command("squashmigrations", "migrations", "0002", interactive=False, verbosity=1, stdout=out)
-        self.assertIn("Optimized from 7 operations to 5 operations.", force_text(out.getvalue()))
+        self.assertIn("Optimized from 7 operations to 3 operations.", force_text(out.getvalue()))
 
     def test_ticket_23799_squashmigrations_no_optimize(self):
         """
diff --git a/tests/migrations/test_optimizer.py b/tests/migrations/test_optimizer.py
index faf297b5e7..dccb783ac7 100644
--- a/tests/migrations/test_optimizer.py
+++ b/tests/migrations/test_optimizer.py
@@ -29,6 +29,9 @@ class OptimizerTests(SimpleTestCase):
         if less_than is not None and iterations >= less_than:
             raise self.failureException("Optimization did not take less than %s iterations (it took %s)" % (less_than, iterations))
 
+    def assertDoesNotOptimize(self, operations):
+        self.assertOptimizesTo(operations, operations)
+
     def test_single(self):
         """
         Tests that the optimizer does nothing on a single operation,
@@ -447,6 +450,155 @@ class OptimizerTests(SimpleTestCase):
             ],
         )
 
+    def _test_create_alter_foo_field(self, alter):
+        """
+        CreateModel, AlterFooTogether/AlterOrderWithRespectTo followed by an
+        add/alter/rename field should optimize to CreateModel and the Alter*
+        """
+        # AddField
+        self.assertOptimizesTo(
+            [
+                migrations.CreateModel("Foo", [
+                    ("a", models.IntegerField()),
+                    ("b", models.IntegerField()),
+                ]),
+                alter,
+                migrations.AddField("Foo", "c", models.IntegerField()),
+            ],
+            [
+                migrations.CreateModel("Foo", [
+                    ("a", models.IntegerField()),
+                    ("b", models.IntegerField()),
+                    ("c", models.IntegerField()),
+                ]),
+                alter,
+            ],
+        )
+
+        # AlterField
+        self.assertDoesNotOptimize(
+            [
+                migrations.CreateModel("Foo", [
+                    ("a", models.IntegerField()),
+                    ("b", models.IntegerField()),
+                ]),
+                alter,
+                migrations.AlterField("Foo", "b", models.CharField(max_length=255)),
+            ],
+        )
+
+        self.assertOptimizesTo(
+            [
+                migrations.CreateModel("Foo", [
+                    ("a", models.IntegerField()),
+                    ("b", models.IntegerField()),
+                    ("c", models.IntegerField()),
+                ]),
+                alter,
+                migrations.AlterField("Foo", "c", models.CharField(max_length=255)),
+            ],
+            [
+                migrations.CreateModel("Foo", [
+                    ("a", models.IntegerField()),
+                    ("b", models.IntegerField()),
+                    ("c", models.CharField(max_length=255)),
+                ]),
+                alter,
+            ],
+        )
+
+        # RenameField
+        self.assertDoesNotOptimize(
+            [
+                migrations.CreateModel("Foo", [
+                    ("a", models.IntegerField()),
+                    ("b", models.IntegerField()),
+                ]),
+                alter,
+                migrations.RenameField("Foo", "b", "c"),
+            ],
+        )
+
+        self.assertOptimizesTo(
+            [
+                migrations.CreateModel("Foo", [
+                    ("a", models.IntegerField()),
+                    ("b", models.IntegerField()),
+                ]),
+                alter,
+                migrations.RenameField("Foo", "b", "x"),
+                migrations.RenameField("Foo", "x", "c"),
+            ],
+            [
+                migrations.CreateModel("Foo", [
+                    ("a", models.IntegerField()),
+                    ("b", models.IntegerField()),
+                ]),
+                alter,
+                migrations.RenameField("Foo", "b", "c"),
+            ],
+        )
+
+        self.assertOptimizesTo(
+            [
+                migrations.CreateModel("Foo", [
+                    ("a", models.IntegerField()),
+                    ("b", models.IntegerField()),
+                    ("c", models.IntegerField()),
+                ]),
+                alter,
+                migrations.RenameField("Foo", "c", "d"),
+            ],
+            [
+                migrations.CreateModel("Foo", [
+                    ("a", models.IntegerField()),
+                    ("b", models.IntegerField()),
+                    ("d", models.IntegerField()),
+                ]),
+                alter,
+            ],
+        )
+
+        # RemoveField
+        self.assertDoesNotOptimize(
+            [
+                migrations.CreateModel("Foo", [
+                    ("a", models.IntegerField()),
+                    ("b", models.IntegerField()),
+                ]),
+                alter,
+                migrations.RemoveField("Foo", "b"),
+            ],
+        )
+
+        self.assertOptimizesTo(
+            [
+                migrations.CreateModel("Foo", [
+                    ("a", models.IntegerField()),
+                    ("b", models.IntegerField()),
+                    ("c", models.IntegerField()),
+                ]),
+                alter,
+                migrations.RemoveField("Foo", "c"),
+            ],
+            [
+                migrations.CreateModel("Foo", [
+                    ("a", models.IntegerField()),
+                    ("b", models.IntegerField()),
+                ]),
+                alter,
+            ],
+        )
+
+    def test_create_alter_unique_field(self):
+        self._test_create_alter_foo_field(migrations.AlterUniqueTogether("Foo", [["a", "b"]]))
+
+    def test_create_alter_index_field(self):
+        self._test_create_alter_foo_field(migrations.AlterIndexTogether("Foo", [["a", "b"]]))
+
+    def test_create_alter_owrt_field(self):
+        self._test_create_alter_foo_field(migrations.AlterOrderWithRespectTo("Foo", "b"))
+
     def test_optimize_through_fields(self):
         """
         Checks that field-level through checking is working.