Fixed #26064 -- Moved operation reduction logic to their own class.

Thanks to Markus Holtermann and Tim Graham for their review.

django/db/migrations/operations/base.py

@@ -111,6 +111,14 @@ class Operation(object):
 
         return router.allow_migrate_model(connection_alias, model)
 
+    def reduce(self, operation, in_between, app_label=None):
+        """
+        Return either a list of operations the actual operation should be
+        replaced with or a boolean that indicates whether or not the specified
+        operation can be optimized across.
+        """
+        return False
+
     def __repr__(self):
         return "<%s %s%s>" % (
             self.__class__.__name__,

django/db/migrations/operations/fields.py

@@ -7,24 +7,38 @@ from django.utils.functional import cached_property
 from .base import Operation
 
 
-class AddField(Operation):
-    """
-    Adds a field to a model.
-    """
-
-    def __init__(self, model_name, name, field, preserve_default=True):
+class FieldOperation(Operation):
+    def __init__(self, model_name, name):
         self.model_name = model_name
         self.name = name
-        self.field = field
-        self.preserve_default = preserve_default
+
+    @cached_property
+    def model_name_lower(self):
+        return self.model_name.lower()
 
     @cached_property
     def name_lower(self):
         return self.name.lower()
 
-    @cached_property
-    def model_name_lower(self):
-        return self.model_name.lower()
+    def is_same_model_operation(self, operation):
+        return self.model_name_lower == operation.model_name_lower
+
+    def is_same_field_operation(self, operation):
+        return self.is_same_model_operation(operation) and self.name_lower == operation.name_lower
+
+    def reduce(self, operation, in_between, app_label=None):
+        return not operation.references_field(self.model_name, self.name, app_label)
+
+
+class AddField(FieldOperation):
+    """
+    Adds a field to a model.
+    """
+
+    def __init__(self, model_name, name, field, preserve_default=True):
+        self.field = field
+        self.preserve_default = preserve_default
+        super(AddField, self).__init__(model_name, name)
 
     def deconstruct(self):
         kwargs = {
@@ -78,24 +92,34 @@ class AddField(Operation):
     def references_field(self, model_name, name, app_label=None):
         return self.references_model(model_name) and name.lower() == self.name_lower
 
+    def reduce(self, operation, in_between, app_label=None):
+        if isinstance(operation, FieldOperation) and self.is_same_field_operation(operation):
+            if isinstance(operation, AlterField):
+                return [
+                    AddField(
+                        model_name=self.model_name,
+                        name=operation.name,
+                        field=operation.field,
+                    ),
+                ]
+            elif isinstance(operation, RemoveField):
+                return []
+            elif isinstance(operation, RenameField):
+                return [
+                    AddField(
+                        model_name=self.model_name,
+                        name=operation.new_name,
+                        field=self.field,
+                    ),
+                ]
+        return super(AddField, self).reduce(operation, in_between, app_label=app_label)
+
 
-class RemoveField(Operation):
+class RemoveField(FieldOperation):
     """
     Removes a field from a model.
     """
 
-    def __init__(self, model_name, name):
-        self.model_name = model_name
-        self.name = name
-
-    @cached_property
-    def name_lower(self):
-        return self.name.lower()
-
-    @cached_property
-    def model_name_lower(self):
-        return self.model_name.lower()
-
     def deconstruct(self):
         kwargs = {
             'model_name': self.model_name,
@@ -136,24 +160,15 @@ class RemoveField(Operation):
         return self.references_model(model_name) and name.lower() == self.name_lower
 
 
-class AlterField(Operation):
+class AlterField(FieldOperation):
     """
     Alters a field's database column (e.g. null, max_length) to the provided new field
     """
 
     def __init__(self, model_name, name, field, preserve_default=True):
-        self.model_name = model_name
-        self.name = name
         self.field = field
         self.preserve_default = preserve_default
-
-    @cached_property
-    def name_lower(self):
-        return self.name.lower()
-
-    @cached_property
-    def model_name_lower(self):
-        return self.model_name.lower()
+        super(AlterField, self).__init__(model_name, name)
 
     def deconstruct(self):
         kwargs = {
@@ -214,16 +229,30 @@ class AlterField(Operation):
     def references_field(self, model_name, name, app_label=None):
         return self.references_model(model_name) and name.lower() == self.name_lower
 
+    def reduce(self, operation, in_between, app_label=None):
+        if isinstance(operation, RemoveField) and self.is_same_field_operation(operation):
+            return [operation]
+        elif isinstance(operation, RenameField) and self.is_same_field_operation(operation):
+            return [
+                operation,
+                AlterField(
+                    model_name=self.model_name,
+                    name=operation.new_name,
+                    field=self.field,
+                ),
+            ]
+        return super(AlterField, self).reduce(operation, in_between, app_label=app_label)
+
 
-class RenameField(Operation):
+class RenameField(FieldOperation):
     """
     Renames a field on the model. Might affect db_column too.
     """
 
     def __init__(self, model_name, old_name, new_name):
-        self.model_name = model_name
         self.old_name = old_name
         self.new_name = new_name
+        super(RenameField, self).__init__(model_name, old_name)
 
     @cached_property
     def old_name_lower(self):
@@ -233,10 +262,6 @@ class RenameField(Operation):
     def new_name_lower(self):
         return self.new_name.lower()
 
-    @cached_property
-    def model_name_lower(self):
-        return self.model_name.lower()
-
     def deconstruct(self):
         kwargs = {
             'model_name': self.model_name,
@@ -296,3 +321,16 @@ class RenameField(Operation):
             name.lower() == self.old_name_lower or
             name.lower() == self.new_name_lower
         )
+
+    def reduce(self, operation, in_between, app_label=None):
+        if (isinstance(operation, RenameField) and
+                self.is_same_model_operation(operation) and
+                self.new_name_lower == operation.old_name_lower):
+            return [
+                RenameField(
+                    self.model_name,
+                    self.old_name,
+                    operation.new_name,
+                ),
+            ]
+        return not operation.references_field(self.model_name, self.new_name, app_label)