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.

2 files changed, 51 insertions, 0 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):