Improve migration optimizer to be able to optimize through other ops

django/db/migrations/operations/base.py

@@ -64,6 +64,18 @@ class Operation(object):
         """
         return "%s: %s" % (self.__class__.__name__, self._constructor_args)
 
+    def references_model(self, name, app_label=None):
+        """
+        Returns True if there is a chance this operation references the given
+        model name (as a string), with an optional app label for accuracy.
+
+        Used for optimization. If in doubt, return True;
+        returning a false positive will merely make the optimizer a little
+        less efficient, while returning a false negative may result in an
+        unusable optimized migration.
+        """
+        return True
+
     def __repr__(self):
         return "<%s %s%s>" % (
             self.__class__.__name__,

django/db/migrations/operations/models.py

@@ -1,4 +1,5 @@
 from .base import Operation
+from django.utils import six
 from django.db import models, router
 from django.db.models.options import normalize_unique_together
 from django.db.migrations.state import ModelState
@@ -33,6 +34,23 @@ class CreateModel(Operation):
     def describe(self):
         return "Create model %s" % (self.name, )
 
+    def references_model(self, name, app_label=None):
+        strings_to_check = [self.name]
+        # Check we didn't inherit from the model
+        for base in self.bases:
+            if isinstance(base, six.string_types):
+                strings_to_check.append(base.split(".")[-1])
+        # Check we have no FKs/M2Ms with it
+        for fname, field in self.fields:
+            if field.rel:
+                if isinstance(field.rel.to, six.string_types):
+                    strings_to_check.append(field.rel.to.split(".")[-1])
+        # Now go over all the strings and compare them
+        for string in strings_to_check:
+            if string.lower() == name.lower():
+                return True
+        return False
+
     def __eq__(self, other):
         return (
             (self.__class__ == other.__class__) and
@@ -66,6 +84,9 @@ class DeleteModel(Operation):
         if router.allow_migrate(schema_editor.connection.alias, model):
             schema_editor.create_model(model)
 
+    def references_model(self, name, app_label=None):
+        return name.lower() == self.name.lower()
+
     def describe(self):
         return "Delete model %s" % (self.name, )
 
@@ -97,6 +118,9 @@ class AlterModelTable(Operation):
     def database_backwards(self, app_label, schema_editor, from_state, to_state):
         return self.database_forwards(app_label, schema_editor, from_state, to_state)
 
+    def references_model(self, name, app_label=None):
+        return name.lower() == self.name.lower()
+
     def describe(self):
         return "Rename table for %s to %s" % (self.name, self.table)
 
@@ -131,6 +155,9 @@ class AlterUniqueTogether(Operation):
     def database_backwards(self, app_label, schema_editor, from_state, to_state):
         return self.database_forwards(app_label, schema_editor, from_state, to_state)
 
+    def references_model(self, name, app_label=None):
+        return name.lower() == self.name.lower()
+
     def describe(self):
         return "Alter unique_together for %s (%s constraints)" % (self.name, len(self.unique_together))
 
@@ -164,5 +191,8 @@ class AlterIndexTogether(Operation):
     def database_backwards(self, app_label, schema_editor, from_state, to_state):
         return self.database_forwards(app_label, schema_editor, from_state, to_state)
 
+    def references_model(self, name, app_label=None):
+        return name.lower() == self.name.lower()
+
     def describe(self):
         return "Alter index_together for %s (%s constraints)" % (self.name, len(self.index_together))

django/db/migrations/optimizer.py

@@ -11,7 +11,7 @@ class MigrationOptimizer(object):
     nothing.
     """
 
-    def optimize(self, operations):
+    def optimize(self, operations, app_label=None):
         """
         Main optimization entry point. Pass in a list of Operation instances,
         get out a new list of Operation instances.
@@ -27,17 +27,20 @@ class MigrationOptimizer(object):
         The inner loop is run until the starting list is the same as the result
         list, and then the result is returned. This means that operation
         optimization must be stable and always return an equal or shorter list.
+
+        The app_label argument is optional, but if you pass it you'll get more
+        efficient optimization.
         """
         # Internal tracking variable for test assertions about # of loops
         self._iterations = 0
         while True:
-            result = self.optimize_inner(operations)
+            result = self.optimize_inner(operations, app_label)
             self._iterations += 1
             if result == operations:
                 return result
             operations = result
 
-    def optimize_inner(self, operations):
+    def optimize_inner(self, operations, app_label=None):
         """
         Inner optimization loop.
         """
@@ -52,7 +55,7 @@ class MigrationOptimizer(object):
                     new_operations.extend(operations[i+1:i+1+j])
                     new_operations.extend(operations[i+j+2:])
                     return new_operations
-                if not self.can_optimize_through(operation, other):
+                if not self.can_optimize_through(operation, other, app_label):
                     new_operations.append(operation)
                     break
             else:
@@ -95,10 +98,22 @@ class MigrationOptimizer(object):
 
     #### THROUGH CHECKS ####
 
-    def can_optimize_through(self, operation, other):
+    def can_optimize_through(self, operation, other, app_label=None):
         """
         Returns True if it's possible to optimize 'operation' with something
         the other side of 'other'. This is possible if, for example, they
         affect different models.
         """
+        MODEL_LEVEL_OPERATIONS = (
+            migrations.CreateModel,
+            migrations.DeleteModel,
+            migrations.AlterModelTable,
+            migrations.AlterUniqueTogether,
+            migrations.AlterIndexTogether,
+        )
+        # If it's a model level operation, let it through if there's
+        # nothing that looks like a reference to us in 'other'.
+        if isinstance(operation, MODEL_LEVEL_OPERATIONS):
+            if not other.references_model(operation.name, app_label):
+                return True
         return False

tests/migrations/test_optimizer.py

@@ -93,3 +93,65 @@ class OptimizerTests(TestCase):
             ],
             [],
         )
+
+    def test_optimize_through_create(self):
+        """
+        We should be able to optimize away create/delete through a create or delete
+        of a different model, but only if the create operation does not mention the model
+        at all.
+        """
+        # These should work
+        self.assertOptimizesTo(
+            [
+                migrations.CreateModel("Foo", [("name", models.CharField(max_length=255))]),
+                migrations.CreateModel("Bar", [("size", models.IntegerField())]),
+                migrations.DeleteModel("Foo"),
+            ],
+            [
+                migrations.CreateModel("Bar", [("size", models.IntegerField())]),
+            ],
+        )
+        self.assertOptimizesTo(
+            [
+                migrations.CreateModel("Foo", [("name", models.CharField(max_length=255))]),
+                migrations.CreateModel("Bar", [("size", models.IntegerField())]),
+                migrations.DeleteModel("Bar"),
+                migrations.DeleteModel("Foo"),
+            ],
+            [],
+        )
+        self.assertOptimizesTo(
+            [
+                migrations.CreateModel("Foo", [("name", models.CharField(max_length=255))]),
+                migrations.CreateModel("Bar", [("size", models.IntegerField())]),
+                migrations.DeleteModel("Foo"),
+                migrations.DeleteModel("Bar"),
+            ],
+            [],
+        )
+        # This should not work - FK should block it
+        self.assertOptimizesTo(
+            [
+                migrations.CreateModel("Foo", [("name", models.CharField(max_length=255))]),
+                migrations.CreateModel("Bar", [("other", models.ForeignKey("testapp.Foo"))]),
+                migrations.DeleteModel("Foo"),
+            ],
+            [
+                migrations.CreateModel("Foo", [("name", models.CharField(max_length=255))]),
+                migrations.CreateModel("Bar", [("other", models.ForeignKey("testapp.Foo"))]),
+                migrations.DeleteModel("Foo"),
+            ],
+        )
+        # This should not work - bases should block it
+        self.assertOptimizesTo(
+            [
+                migrations.CreateModel("Foo", [("name", models.CharField(max_length=255))]),
+                migrations.CreateModel("Bar", [("size", models.IntegerField())], bases=("testapp.Foo", )),
+                migrations.DeleteModel("Foo"),
+            ],
+            [
+                migrations.CreateModel("Foo", [("name", models.CharField(max_length=255))]),
+                migrations.CreateModel("Bar", [("size", models.IntegerField())], bases=("testapp.Foo", )),
+                migrations.DeleteModel("Foo"),
+            ],
+        )