Fixed #24109 -- Allowed RunSQL and RunPython operations to be elided.

Thanks to Markus Holtermann and Tim Graham for their review.

django/db/migrations/operations/base.py

@@ -30,6 +30,9 @@ class Operation(object):
     # DDL transaction support (i.e., does it have no DDL, like RunPython)
     atomic = False
 
+    # Should this operation be considered safe to elide and optimize across?
+    elidable = False
+
     serialization_expand_args = []
 
     def __new__(cls, *args, **kwargs):
@@ -117,6 +120,10 @@ class Operation(object):
         replaced with or a boolean that indicates whether or not the specified
         operation can be optimized across.
         """
+        if self.elidable:
+            return [operation]
+        elif operation.elidable:
+            return [self]
         return False
 
     def __repr__(self):

django/db/migrations/operations/fields.py

@@ -27,7 +27,10 @@ class FieldOperation(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)
+        return (
+            super(FieldOperation, self).reduce(operation, in_between, app_label=app_label) or
+            not operation.references_field(self.model_name, self.name, app_label)
+        )
 
 
 class AddField(FieldOperation):
@@ -333,4 +336,9 @@ class RenameField(FieldOperation):
                     operation.new_name,
                 ),
             ]
-        return not operation.references_field(self.model_name, self.new_name, app_label)
+        # Skip `FieldOperation.reduce` as we want to run `references_field`
+        # against self.new_name.
+        return (
+            super(FieldOperation, self).reduce(operation, in_between, app_label=app_label) or
+            not operation.references_field(self.model_name, self.new_name, app_label)
+        )

django/db/migrations/operations/models.py

@@ -21,7 +21,10 @@ class ModelOperation(Operation):
         return self.name.lower()
 
     def reduce(self, operation, in_between, app_label=None):
-        return not operation.references_model(self.name, app_label)
+        return (
+            super(ModelOperation, self).reduce(operation, in_between, app_label=app_label) or
+            not operation.references_model(self.name, app_label)
+        )
 
 
 class CreateModel(ModelOperation):
@@ -365,7 +368,12 @@ class RenameModel(ModelOperation):
                     operation.new_name,
                 ),
             ]
-        return not operation.references_model(self.new_name, app_label)
+        # Skip `ModelOperation.reduce` as we want to run `references_model`
+        # against self.new_name.
+        return (
+            super(ModelOperation, self).reduce(operation, in_between, app_label=app_label) or
+            not operation.references_model(self.new_name, app_label)
+        )
 
 
 class AlterModelTable(ModelOperation):

django/db/migrations/operations/special.py

@@ -71,11 +71,12 @@ class RunSQL(Operation):
     """
     noop = ''
 
-    def __init__(self, sql, reverse_sql=None, state_operations=None, hints=None):
+    def __init__(self, sql, reverse_sql=None, state_operations=None, hints=None, elidable=False):
         self.sql = sql
         self.reverse_sql = reverse_sql
         self.state_operations = state_operations or []
         self.hints = hints or {}
+        self.elidable = elidable
 
     def deconstruct(self):
         kwargs = {
@@ -138,7 +139,7 @@ class RunPython(Operation):
 
     reduces_to_sql = False
 
-    def __init__(self, code, reverse_code=None, atomic=True, hints=None):
+    def __init__(self, code, reverse_code=None, atomic=True, hints=None, elidable=False):
         self.atomic = atomic
         # Forwards code
         if not callable(code):
@@ -152,6 +153,7 @@ class RunPython(Operation):
                 raise ValueError("RunPython must be supplied with callable arguments")
             self.reverse_code = reverse_code
         self.hints = hints or {}
+        self.elidable = elidable
 
     def deconstruct(self):
         kwargs = {