[1.9.x] Fixed #25807 -- Instructed the migration writer about lazy objects.

Thanks to Trac alias mrgaolei for the report, Baptiste for the confirmation
and Tim for the review.

Backport of cc2ca9c550 from master

django/db/migrations/writer.py

@@ -19,7 +19,7 @@ from django.db.migrations.utils import COMPILED_REGEX_TYPE, RegexObject
 from django.utils import datetime_safe, six
 from django.utils._os import upath
 from django.utils.encoding import force_text
-from django.utils.functional import Promise
+from django.utils.functional import LazyObject, Promise
 from django.utils.inspect import get_func_args
 from django.utils.module_loading import module_dir
 from django.utils.timezone import now, utc
@@ -344,6 +344,10 @@ class MigrationWriter(object):
         # process.
         if isinstance(value, Promise):
             value = force_text(value)
+        elif isinstance(value, LazyObject):
+            # The unwrapped value is returned as the first item of the
+            # arguments tuple.
+            value = value.__reduce__()[1][0]
 
         # Sequences
         if isinstance(value, (frozenset, list, set, tuple)):

docs/releases/1.9.txt

@@ -448,7 +448,8 @@ Migrations
   :djadminopt:`migrate --fake-initial <--fake-initial>` to more easily detect
   initial migrations.
 
-* Added support for serialization of ``functools.partial`` objects.
+* Added support for serialization of ``functools.partial`` and ``LazyObject``
+  instances.
 
 * When supplying ``None`` as a value in :setting:`MIGRATION_MODULES`, Django
   will consider the app an app without migrations.

docs/topics/migrations.txt

@@ -659,6 +659,7 @@ Django can serialize the following:
 - ``decimal.Decimal`` instances
 - ``functools.partial`` instances which have serializable ``func``, ``args``,
   and ``keywords`` values.
+- ``LazyObject`` instances which wrap a serializable value.
 - Any Django field
 - Any function or method reference (e.g. ``datetime.datetime.today``) (must be in module's top-level scope)
 - Any class reference (must be in module's top-level scope)
@@ -666,7 +667,8 @@ Django can serialize the following:
 
 .. versionchanged:: 1.9
 
-    Serialization support for `functools.partial` was added.
+    Serialization support for ``functools.partial`` and ``LazyObject``
+    instances was added.
 
 Django can serialize the following on Python 3 only:
 

tests/migrations/test_writer.py

@@ -23,6 +23,7 @@ from django.test import SimpleTestCase, ignore_warnings, mock
 from django.utils import datetime_safe, six
 from django.utils._os import upath
 from django.utils.deconstruct import deconstructible
+from django.utils.functional import SimpleLazyObject
 from django.utils.timezone import FixedOffset, get_default_timezone, utc
 from django.utils.translation import ugettext_lazy as _
 
@@ -229,6 +230,11 @@ class WriterTests(SimpleTestCase):
             ("[list, tuple, dict, set, frozenset]", set())
         )
 
+    def test_serialize_lazy_objects(self):
+        pattern = re.compile(r'^foo$', re.UNICODE)
+        lazy_pattern = SimpleLazyObject(lambda: pattern)
+        self.assertEqual(self.serialize_round_trip(lazy_pattern), pattern)
+
     def test_serialize_functions(self):
         with six.assertRaisesRegex(self, ValueError, 'Cannot serialize function: lambda'):
             self.assertSerializedEqual(lambda x: 42)