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):
+class FieldOperation(Operation):
-    """
+    def __init__(self, model_name, name):
-    Adds a field to a model.
-    """
-
-    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 model_name_lower(self):
+        return self.model_name.lower()
 
     @cached_property
     def name_lower(self):
         return self.name.lower()
 
-    @cached_property
+    def is_same_model_operation(self, operation):
-    def model_name_lower(self):
+        return self.model_name_lower == operation.model_name_lower
-        return self.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
-
+        super(AlterField, self).__init__(model_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 = {
@@ -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)

django/db/migrations/operations/models.py

@@ -7,8 +7,24 @@ from django.db.models.options import normalize_together
 from django.utils import six
 from django.utils.functional import cached_property
 
+from .fields import (
+    AddField, AlterField, FieldOperation, RemoveField, RenameField,
+)
 
-class CreateModel(Operation):
+
+class ModelOperation(Operation):
+    def __init__(self, name):
+        self.name = name
+
+    @cached_property
+    def name_lower(self):
+        return self.name.lower()
+
+    def reduce(self, operation, in_between, app_label=None):
+        return not operation.references_model(self.name, app_label)
+
+
+class CreateModel(ModelOperation):
     """
     Create a model's table.
     """
@@ -16,15 +32,11 @@ class CreateModel(Operation):
     serialization_expand_args = ['fields', 'options', 'managers']
 
     def __init__(self, name, fields, options=None, bases=None, managers=None):
-        self.name = name
         self.fields = fields
         self.options = options or {}
         self.bases = bases or (models.Model,)
         self.managers = managers or []
-
+        super(CreateModel, self).__init__(name)
-    @cached_property
-    def name_lower(self):
-        return self.name.lower()
 
     def deconstruct(self):
         kwargs = {
@@ -83,19 +95,102 @@ class CreateModel(Operation):
                 return True
         return False
 
+    def model_to_key(self, model):
+        """
+        Take either a model class or an "app_label.ModelName" string
+        and return (app_label, object_name).
+        """
+        if isinstance(model, six.string_types):
+            return model.split(".", 1)
+        else:
+            return model._meta.app_label, model._meta.object_name
 
-class DeleteModel(Operation):
+    def reduce(self, operation, in_between, app_label=None):
+        if (isinstance(operation, DeleteModel) and
+                self.name_lower == operation.name_lower and
+                not self.options.get("proxy", False)):
+            return []
+        elif isinstance(operation, RenameModel) and self.name_lower == operation.old_name_lower:
+            return [
+                CreateModel(
+                    operation.new_name,
+                    fields=self.fields,
+                    options=self.options,
+                    bases=self.bases,
+                    managers=self.managers,
+                ),
+            ]
+        elif isinstance(operation, FieldOperation) and self.name_lower == operation.model_name_lower:
+            if isinstance(operation, AddField):
+                # Don't allow optimizations of FKs through models they reference
+                if hasattr(operation.field, "remote_field") and operation.field.remote_field:
+                    for between in in_between:
+                        # Check that it doesn't point to the model
+                        app_label, object_name = self.model_to_key(operation.field.remote_field.model)
+                        if between.references_model(object_name, app_label):
+                            return False
+                        # Check that it's not through the model
+                        if getattr(operation.field.remote_field, "through", None):
+                            app_label, object_name = self.model_to_key(operation.field.remote_field.through)
+                            if between.references_model(object_name, app_label):
+                                return False
+                return [
+                    CreateModel(
+                        self.name,
+                        fields=self.fields + [(operation.name, operation.field)],
+                        options=self.options,
+                        bases=self.bases,
+                        managers=self.managers,
+                    ),
+                ]
+            elif isinstance(operation, AlterField):
+                return [
+                    CreateModel(
+                        self.name,
+                        fields=[
+                            (n, operation.field if n == operation.name else v)
+                            for n, v in self.fields
+                        ],
+                        options=self.options,
+                        bases=self.bases,
+                        managers=self.managers,
+                    ),
+                ]
+            elif isinstance(operation, RemoveField):
+                return [
+                    CreateModel(
+                        self.name,
+                        fields=[
+                            (n, v)
+                            for n, v in self.fields
+                            if n.lower() != operation.name_lower
+                        ],
+                        options=self.options,
+                        bases=self.bases,
+                        managers=self.managers,
+                    ),
+                ]
+            elif isinstance(operation, RenameField):
+                return [
+                    CreateModel(
+                        self.name,
+                        fields=[
+                            (operation.new_name if n == operation.old_name else n, v)
+                            for n, v in self.fields
+                        ],
+                        options=self.options,
+                        bases=self.bases,
+                        managers=self.managers,
+                    ),
+                ]
+        return super(CreateModel, self).reduce(operation, in_between, app_label=app_label)
+
+
+class DeleteModel(ModelOperation):
     """
     Drops a model's table.
     """
 
-    def __init__(self, name):
-        self.name = name
-
-    @cached_property
-    def name_lower(self):
-        return self.name.lower()
-
     def deconstruct(self):
         kwargs = {
             'name': self.name,
@@ -126,7 +221,7 @@ class DeleteModel(Operation):
         return "Delete model %s" % (self.name, )
 
 
-class RenameModel(Operation):
+class RenameModel(ModelOperation):
     """
     Renames a model.
     """
@@ -134,6 +229,7 @@ class RenameModel(Operation):
     def __init__(self, old_name, new_name):
         self.old_name = old_name
         self.new_name = new_name
+        super(RenameModel, self).__init__(old_name)
 
     @cached_property
     def old_name_lower(self):
@@ -260,19 +356,26 @@ class RenameModel(Operation):
     def describe(self):
         return "Rename model %s to %s" % (self.old_name, self.new_name)
 
+    def reduce(self, operation, in_between, app_label=None):
+        if (isinstance(operation, RenameModel) and
+                self.new_name_lower == operation.old_name_lower):
+            return [
+                RenameModel(
+                    self.old_name,
+                    operation.new_name,
+                ),
+            ]
+        return not operation.references_model(self.new_name, app_label)
 
-class AlterModelTable(Operation):
+
+class AlterModelTable(ModelOperation):
     """
     Renames a model's table
     """
 
     def __init__(self, name, table):
-        self.name = name
         self.table = table
-
+        super(AlterModelTable, self).__init__(name)
-    @cached_property
-    def name_lower(self):
-        return self.name.lower()
 
     def deconstruct(self):
         kwargs = {
@@ -316,8 +419,29 @@ class AlterModelTable(Operation):
     def describe(self):
         return "Rename table for %s to %s" % (self.name, self.table)
 
+    def reduce(self, operation, in_between, app_label=None):
+        if isinstance(operation, (AlterModelTable, DeleteModel)) and self.name_lower == operation.name_lower:
+            return [operation]
+        return super(AlterModelTable, self).reduce(operation, in_between, app_label=app_label)
 
-class AlterUniqueTogether(Operation):
+
+class ModelOptionOperation(ModelOperation):
+    def reduce(self, operation, in_between, app_label=None):
+        if isinstance(operation, (self.__class__, DeleteModel)) and self.name_lower == operation.name_lower:
+            return [operation]
+        return super(ModelOptionOperation, self).reduce(operation, in_between, app_label=app_label)
+
+
+class FieldRelatedOptionOperation(ModelOptionOperation):
+    def reduce(self, operation, in_between, app_label=None):
+        if (isinstance(operation, FieldOperation) and
+                self.name_lower == operation.model_name_lower and
+                not self.references_field(operation.model_name, operation.name)):
+            return [operation, self]
+        return super(FieldRelatedOptionOperation, self).reduce(operation, in_between, app_label=app_label)
+
+
+class AlterUniqueTogether(FieldRelatedOptionOperation):
     """
     Changes the value of unique_together to the target one.
     Input value of unique_together must be a set of tuples.
@@ -325,13 +449,9 @@ class AlterUniqueTogether(Operation):
     option_name = "unique_together"
 
     def __init__(self, name, unique_together):
-        self.name = name
         unique_together = normalize_together(unique_together)
         self.unique_together = set(tuple(cons) for cons in unique_together)
-
+        super(AlterUniqueTogether, self).__init__(name)
-    @cached_property
-    def name_lower(self):
-        return self.name.lower()
 
     def deconstruct(self):
         kwargs = {
@@ -378,7 +498,7 @@ class AlterUniqueTogether(Operation):
         return "Alter %s for %s (%s constraint(s))" % (self.option_name, self.name, len(self.unique_together or ''))
 
 
-class AlterIndexTogether(Operation):
+class AlterIndexTogether(FieldRelatedOptionOperation):
     """
     Changes the value of index_together to the target one.
     Input value of index_together must be a set of tuples.
@@ -386,13 +506,9 @@ class AlterIndexTogether(Operation):
     option_name = "index_together"
 
     def __init__(self, name, index_together):
-        self.name = name
         index_together = normalize_together(index_together)
         self.index_together = set(tuple(cons) for cons in index_together)
-
+        super(AlterIndexTogether, self).__init__(name)
-    @cached_property
-    def name_lower(self):
-        return self.name.lower()
 
     def deconstruct(self):
         kwargs = {
@@ -439,18 +555,14 @@ class AlterIndexTogether(Operation):
         return "Alter %s for %s (%s constraint(s))" % (self.option_name, self.name, len(self.index_together or ''))
 
 
-class AlterOrderWithRespectTo(Operation):
+class AlterOrderWithRespectTo(FieldRelatedOptionOperation):
     """
     Represents a change with the order_with_respect_to option.
     """
 
     def __init__(self, name, order_with_respect_to):
-        self.name = name
         self.order_with_respect_to = order_with_respect_to
-
+        super(AlterOrderWithRespectTo, self).__init__(name)
-    @cached_property
-    def name_lower(self):
-        return self.name.lower()
 
     def deconstruct(self):
         kwargs = {
@@ -505,7 +617,7 @@ class AlterOrderWithRespectTo(Operation):
         return "Set order_with_respect_to on %s to %s" % (self.name, self.order_with_respect_to)
 
 
-class AlterModelOptions(Operation):
+class AlterModelOptions(ModelOptionOperation):
     """
     Sets new model options that don't directly affect the database schema
     (like verbose_name, permissions, ordering). Python code in migrations
@@ -525,12 +637,8 @@ class AlterModelOptions(Operation):
     ]
 
     def __init__(self, name, options):
-        self.name = name
         self.options = options
-
+        super(AlterModelOptions, self).__init__(name)
-    @cached_property
-    def name_lower(self):
-        return self.name.lower()
 
     def deconstruct(self):
         kwargs = {
@@ -565,7 +673,7 @@ class AlterModelOptions(Operation):
         return "Change Meta options on %s" % (self.name, )
 
 
-class AlterModelManagers(Operation):
+class AlterModelManagers(ModelOptionOperation):
     """
     Alters the model's managers
     """
@@ -573,12 +681,8 @@ class AlterModelManagers(Operation):
     serialization_expand_args = ['managers']
 
     def __init__(self, name, managers):
-        self.name = name
         self.managers = managers
-
+        super(AlterModelManagers, self).__init__(name)
-    @cached_property
-    def name_lower(self):
-        return self.name.lower()
 
     def deconstruct(self):
         return (

django/db/migrations/optimizer.py

@@ -1,12 +1,5 @@
 from __future__ import unicode_literals
 
-from django.db.migrations import (
-    AddField, AlterField, AlterIndexTogether, AlterModelTable,
-    AlterOrderWithRespectTo, AlterUniqueTogether, CreateModel, DeleteModel,
-    RemoveField, RenameField, RenameModel,
-)
-from django.utils import six
-
 
 class MigrationOptimizer(object):
     """
@@ -19,63 +12,6 @@ class MigrationOptimizer(object):
     nothing.
     """
 
-    def __init__(self):
-        self.model_level_operations = (
-            CreateModel,
-            AlterModelTable,
-            AlterUniqueTogether,
-            AlterIndexTogether,
-            AlterOrderWithRespectTo,
-        )
-        self.field_level_operations = (
-            AddField,
-            AlterField,
-        )
-        self.reduce_methods = {
-            # (model operation, model operation)
-            (CreateModel, DeleteModel): self.reduce_create_model_delete_model,
-            (CreateModel, RenameModel): self.reduce_create_model_rename_model,
-            (RenameModel, RenameModel): self.reduce_rename_model_rename_model,
-
-            (AlterIndexTogether, AlterIndexTogether): self.reduce_alter_model_alter_model,
-            (AlterModelTable, AlterModelTable): self.reduce_alter_model_alter_model,
-            (AlterOrderWithRespectTo, AlterOrderWithRespectTo): self.reduce_alter_model_alter_model,
-            (AlterUniqueTogether, AlterUniqueTogether): self.reduce_alter_model_alter_model,
-
-            (AlterIndexTogether, DeleteModel): self.reduce_alter_model_delete_model,
-            (AlterModelTable, DeleteModel): self.reduce_alter_model_delete_model,
-            (AlterOrderWithRespectTo, DeleteModel): self.reduce_alter_model_delete_model,
-            (AlterUniqueTogether, DeleteModel): self.reduce_alter_model_delete_model,
-
-            # (model operation, field operation)
-            (CreateModel, AddField): self.reduce_create_model_add_field,
-            (CreateModel, AlterField): self.reduce_create_model_alter_field,
-            (CreateModel, RemoveField): self.reduce_create_model_remove_field,
-            (CreateModel, RenameField): self.reduce_create_model_rename_field,
-
-            (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,
-
-            # (field operation, field operation)
-            (AddField, AlterField): self.reduce_add_field_alter_field,
-            (AddField, RemoveField): self.reduce_add_field_remove_field,
-            (AddField, RenameField): self.reduce_add_field_rename_field,
-            (AlterField, RemoveField): self.reduce_alter_field_remove_field,
-            (AlterField, RenameField): self.reduce_alter_field_rename_field,
-            (RenameField, RenameField): self.reduce_rename_field_rename_field,
-        }
-
     def optimize(self, operations, app_label=None):
         """
         Main optimization entry point. Pass in a list of Operation instances,
@@ -113,253 +49,18 @@ class MigrationOptimizer(object):
         for i, operation in enumerate(operations):
             # Compare it to each operation after it
             for j, other in enumerate(operations[i + 1:]):
-                result = self.reduce(operation, other, operations[i + 1:i + j + 1])
+                in_between = operations[i + 1:i + j + 1]
-                if result is not None:
+                result = operation.reduce(other, in_between, app_label)
+                if isinstance(result, list):
                     # Optimize! Add result, then remaining others, then return
                     new_operations.extend(result)
-                    new_operations.extend(operations[i + 1:i + 1 + j])
+                    new_operations.extend(in_between)
                     new_operations.extend(operations[i + j + 2:])
                     return new_operations
-                if not self.can_optimize_through(operation, other, app_label):
+                if not result:
+                    # We can't optimize across `other`.
                     new_operations.append(operation)
                     break
             else:
                 new_operations.append(operation)
         return new_operations
-
-    # REDUCTION
-
-    def reduce(self, operation, other, in_between=None):
-        """
-        Either returns a list of zero, one or two operations,
-        or None, meaning this pair cannot be optimized.
-        """
-        method = self.reduce_methods.get((type(operation), type(other)))
-        if method:
-            return method(operation, other, in_between or [])
-        return None
-
-    def model_to_key(self, model):
-        """
-        Takes either a model class or a "appname.ModelName" string
-        and returns (appname, modelname)
-        """
-        if isinstance(model, six.string_types):
-            return model.split(".", 1)
-        else:
-            return (
-                model._meta.app_label,
-                model._meta.object_name,
-            )
-
-    # REDUCE METHODS: (MODEL OPERATION, MODEL OPERATION)
-
-    def reduce_create_model_delete_model(self, operation, other, in_between):
-        """
-        Folds a CreateModel and a DeleteModel into nothing.
-        """
-        if (operation.name_lower == other.name_lower and
-                not operation.options.get("proxy", False)):
-            return []
-
-    def reduce_create_model_rename_model(self, operation, other, in_between):
-        """
-        Folds a model rename into its create
-        """
-        if operation.name_lower == other.old_name_lower:
-            return [
-                CreateModel(
-                    other.new_name,
-                    fields=operation.fields,
-                    options=operation.options,
-                    bases=operation.bases,
-                    managers=operation.managers,
-                )
-            ]
-
-    def reduce_rename_model_rename_model(self, operation, other, in_between):
-        """
-        Folds a model rename into another one
-        """
-        if operation.new_name_lower == other.old_name_lower:
-            return [
-                RenameModel(
-                    operation.old_name,
-                    other.new_name,
-                )
-            ]
-
-    def reduce_alter_model_alter_model(self, operation, other, in_between):
-        """
-        Folds two AlterModelTable, AlterFooTogether, or AlterOrderWithRespectTo
-        operations into the latter.
-        """
-        if operation.name_lower == other.name_lower:
-            return [other]
-
-    def reduce_alter_model_delete_model(self, operation, other, in_between):
-        """
-        Folds an AlterModelSomething and a DeleteModel into just delete.
-        """
-        if operation.name_lower == other.name_lower:
-            return [other]
-
-    # REDUCE METHODS: (MODEL OPERATION, FIELD OPERATION)
-
-    def reduce_create_model_add_field(self, operation, other, in_between):
-        if operation.name_lower == other.model_name_lower:
-            # Don't allow optimizations of FKs through models they reference
-            if hasattr(other.field, "remote_field") and other.field.remote_field:
-                for between in in_between:
-                    # Check that it doesn't point to the model
-                    app_label, object_name = self.model_to_key(other.field.remote_field.model)
-                    if between.references_model(object_name, app_label):
-                        return None
-                    # Check that it's not through the model
-                    if getattr(other.field.remote_field, "through", None):
-                        app_label, object_name = self.model_to_key(other.field.remote_field.through)
-                        if between.references_model(object_name, app_label):
-                            return None
-            # OK, that's fine
-            return [
-                CreateModel(
-                    operation.name,
-                    fields=operation.fields + [(other.name, other.field)],
-                    options=operation.options,
-                    bases=operation.bases,
-                    managers=operation.managers,
-                )
-            ]
-
-    def reduce_create_model_alter_field(self, operation, other, in_between):
-        if operation.name_lower == other.model_name_lower:
-            return [
-                CreateModel(
-                    operation.name,
-                    fields=[
-                        (n, other.field if n == other.name else v)
-                        for n, v in operation.fields
-                    ],
-                    options=operation.options,
-                    bases=operation.bases,
-                    managers=operation.managers,
-                )
-            ]
-
-    def reduce_create_model_remove_field(self, operation, other, in_between):
-        if operation.name_lower == other.model_name_lower:
-            return [
-                CreateModel(
-                    operation.name,
-                    fields=[
-                        (n, v)
-                        for n, v in operation.fields
-                        if n.lower() != other.name_lower
-                    ],
-                    options=operation.options,
-                    bases=operation.bases,
-                    managers=operation.managers,
-                )
-            ]
-
-    def reduce_create_model_rename_field(self, operation, other, in_between):
-        if operation.name_lower == other.model_name_lower:
-            return [
-                CreateModel(
-                    operation.name,
-                    fields=[
-                        (other.new_name if n == other.old_name else n, v)
-                        for n, v in operation.fields
-                    ],
-                    options=operation.options,
-                    bases=operation.bases,
-                    managers=operation.managers,
-                )
-            ]
-
-    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]
-
-    # REDUCE METHODS: (FIELD OPERATION, FIELD OPERATION)
-
-    def reduce_add_field_alter_field(self, operation, other, in_between):
-        if (operation.model_name_lower == other.model_name_lower and
-                operation.name_lower == other.name_lower):
-            return [
-                AddField(
-                    model_name=operation.model_name,
-                    name=operation.name,
-                    field=other.field,
-                )
-            ]
-
-    def reduce_add_field_remove_field(self, operation, other, in_between):
-        if (operation.model_name_lower == other.model_name_lower and
-                operation.name_lower == other.name_lower):
-            return []
-
-    def reduce_add_field_rename_field(self, operation, other, in_between):
-        if (operation.model_name_lower == other.model_name_lower and
-                operation.name_lower == other.old_name_lower):
-            return [
-                AddField(
-                    model_name=operation.model_name,
-                    name=other.new_name,
-                    field=operation.field,
-                )
-            ]
-
-    def reduce_alter_field_remove_field(self, operation, other, in_between):
-        if (operation.model_name_lower == other.model_name_lower and
-                operation.name_lower == other.name_lower):
-            return [other]
-
-    def reduce_alter_field_rename_field(self, operation, other, in_between):
-        if (operation.model_name_lower == other.model_name_lower and
-                operation.name_lower == other.old_name_lower):
-            return [
-                other,
-                AlterField(
-                    model_name=operation.model_name,
-                    name=other.new_name,
-                    field=operation.field,
-                ),
-            ]
-
-    def reduce_rename_field_rename_field(self, operation, other, in_between):
-        if (operation.model_name_lower == other.model_name_lower and
-                operation.new_name_lower == other.old_name_lower):
-            return [
-                RenameField(
-                    operation.model_name,
-                    operation.old_name,
-                    other.new_name,
-                ),
-            ]
-
-    # THROUGH CHECKS
-
-    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.
-        """
-        # 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, self.model_level_operations):
-            if not other.references_model(operation.name, app_label):
-                return True
-        # If it's field level, only let it through things that don't reference
-        # the field (which includes not referencing the model)
-        if isinstance(operation, self.field_level_operations):
-            if not other.references_field(operation.model_name, operation.name, app_label):
-                return True
-        return False