[1.7.x] Fixed #22564 -- Prevented unneeded bytestrings in migrations

In some cases, this could lead to migrations written with Python 2
being incompatible with Python 3.
Thanks Tim Graham for the report and Loïc Bistuer for the advices.
Backport of da9cf53cb from master.

django/db/migrations/state.py

@@ -6,6 +6,7 @@ from django.db import models
 from django.db.models.options import DEFAULT_NAMES, normalize_together
 from django.db.models.fields.related import do_pending_lookups
 from django.utils import six
+from django.utils.encoding import force_text
 from django.utils.module_loading import import_string
 
 
@@ -132,7 +133,7 @@ class ModelState(object):
 
     def __init__(self, app_label, name, fields, options=None, bases=None):
         self.app_label = app_label
-        self.name = name
+        self.name = force_text(name)
         self.fields = fields
         self.options = options or {}
         self.bases = bases or (models.Model, )

django/db/migrations/writer.py

@@ -124,6 +124,8 @@ class MigrationWriter(object):
                 dependencies.append("        migrations.swappable_dependency(settings.%s)," % dependency[1])
                 imports.add("from django.conf import settings")
             else:
+                # No need to output bytestrings for dependencies
+                dependency = tuple([force_text(s) for s in dependency])
                 dependencies.append("        %s," % self.serialize(dependency)[0])
         items["dependencies"] = "\n".join(dependencies) + "\n" if dependencies else ""
 

django/db/models/fields/__init__.py

@@ -370,7 +370,7 @@ class Field(RegisterLookupMixin):
             path = path.replace("django.db.models.fields", "django.db.models")
         # Return basic info - other fields should override this.
         return (
-            self.name,
+            force_text(self.name, strings_only=True),
             path,
             [],
             keywords,

tests/field_deconstruction/tests.py

@@ -1,6 +1,8 @@
 import warnings
-from django.test import TestCase, override_settings
+
 from django.db import models
+from django.test import TestCase, override_settings
+from django.utils import six
 
 
 class FieldDeconstructionTests(TestCase):
@@ -15,14 +17,15 @@ class FieldDeconstructionTests(TestCase):
         # First try using a "normal" field
         field = models.CharField(max_length=65)
         name, path, args, kwargs = field.deconstruct()
-        self.assertEqual(name, None)
+        self.assertIsNone(name)
         field.set_attributes_from_name("is_awesome_test")
         name, path, args, kwargs = field.deconstruct()
         self.assertEqual(name, "is_awesome_test")
+        self.assertIsInstance(name, six.text_type)
         # Now try with a ForeignKey
         field = models.ForeignKey("some_fake.ModelName")
         name, path, args, kwargs = field.deconstruct()
-        self.assertEqual(name, None)
+        self.assertIsNone(name)
         field.set_attributes_from_name("author")
         name, path, args, kwargs = field.deconstruct()
         self.assertEqual(name, "author")