Prevent a migration changing DateTimeField(auto_now_add=True) to default=timezone.now from generating SQL

[Дилян Палаузов]

A switch from DateTimeField(auto_now_add=True) to DateTimeField(default=django.utils.timezone.new) creates the statements

ALTER TABLE SET DEFAULT '2017-10-16T09:35:52.710695'::timestamp;
ALTER TABLE DROP DEFAULT

which have no effects, apart from locking the whole table twice.

A proposal to recognize, when the effective default-callable doesn't change and skip changing the DEFAULT twice in this case, as well as not generating a migration when this is the only change on a field:

diff --git a/django/db/backends/base/schema.py b/django/db/backends/base/schema.py
--- a/django/db/backends/base/schema.py
+++ b/django/db/backends/base/schema.py
@@ -199,28 +199,33 @@ class BaseDatabaseSchemaEditor(object):
             'requires_literal_defaults must provide a prepare_default() method'
         )
 
-    def effective_default(self, field):
+    @staticmethod
+    def effective_default_before_callable(field):
         """
-        Returns a field's effective database default value
+        Returns a field's effective database default callable or value
         """
         if field.has_default():
-            default = field.get_default()
+            return field._get_default
         elif not field.null and field.blank and field.empty_strings_allowed:
             if field.get_internal_type() == "BinaryField":
-                default = six.binary_type()
+                return six.binary_type()
             else:
-                default = six.text_type()
+                return six.text_type()
         elif getattr(field, 'auto_now', False) or getattr(field, 'auto_now_add', False):
             default = datetime.now()
             internal_type = field.get_internal_type()
             if internal_type == 'DateField':
-                default = default.date
+                return default.date
             elif internal_type == 'TimeField':
-                default = default.time
+                return default.time
             elif internal_type == 'DateTimeField':
-                default = timezone.now
-        else:
-            default = None
+                return timezone.now
+
+    def effective_default(self, field):
+        """
+        Returns a field's effective database default value
+        """
+        default = BaseDatabaseSchemaEditor.effective_default_before_callable(field)
         # If it's a callable, call it
         if callable(default):
             default = default()
@@ -615,6 +620,7 @@ class BaseDatabaseSchemaEditor(object):
             old_default != new_default and
             new_default is not None and
             not self.skip_default(new_field)
+            and BaseDatabaseSchemaEditor.effective_default_before_callable(old_field) != BaseDatabaseSchemaEdit
         )
         if needs_database_default:
             if self.connection.features.requires_literal_defaults:
diff --git a/django/db/models/fields/__init__.py b/django/db/models/fields/__init__.py
--- a/django/db/models/fields/__init__.py
+++ b/django/db/models/fields/__init__.py
@@ -1232,7 +1232,7 @@ class DateField(DateTimeCheckMixin, Field):
         if self.auto_now:
             kwargs['auto_now'] = True
         if self.auto_now_add:
-            kwargs['auto_now_add'] = True
+            kwargs['default'] = timezone.now
         if self.auto_now or self.auto_now_add:
             del kwargs['editable']
             del kwargs['blank']