Fixed #22474 -- Made migration recorder aware of multiple databases

Thanks Tim Graham for the review.

django/db/migrations/recorder.py

 from django.apps.registry import Apps
 from django.db import models
+from django.utils.encoding import python_2_unicode_compatible
 from django.utils.timezone import now
 
 
@@ -16,6 +17,7 @@ class MigrationRecorder(object):
     a row in the table always means a migration is applied.
     """
 
+    @python_2_unicode_compatible
     class Migration(models.Model):
         app = models.CharField(max_length=255)
         name = models.CharField(max_length=255)
@@ -26,9 +28,16 @@ class MigrationRecorder(object):
             app_label = "migrations"
             db_table = "django_migrations"
 
+        def __str__(self):
+            return "Migration %s for %s" % (self.name, self.app)
+
     def __init__(self, connection):
         self.connection = connection
 
+    @property
+    def migration_qs(self):
+        return self.Migration.objects.using(self.connection.alias)
+
     def ensure_schema(self):
         """
         Ensures the table exists and has the correct schema.
@@ -46,25 +55,24 @@ class MigrationRecorder(object):
         Returns a set of (app, name) of applied migrations.
         """
         self.ensure_schema()
-        return set(tuple(x) for x in self.Migration.objects.values_list("app", "name"))
+        return set(tuple(x) for x in self.migration_qs.values_list("app", "name"))
 
     def record_applied(self, app, name):
         """
         Records that a migration was applied.
         """
         self.ensure_schema()
-        self.Migration.objects.create(app=app, name=name)
+        self.migration_qs.create(app=app, name=name)
 
     def record_unapplied(self, app, name):
         """
         Records that a migration was unapplied.
         """
         self.ensure_schema()
-        self.Migration.objects.filter(app=app, name=name).delete()
+        self.migration_qs.filter(app=app, name=name).delete()
 
-    @classmethod
-    def flush(cls):
+    def flush(self):
         """
         Deletes all migration records. Useful if you're testing migrations.
         """
-        cls.Migration.objects.all().delete()
+        self.migration_qs.all().delete()

tests/migrations/test_loader.py

 from unittest import skipIf
 
 from django.test import TestCase, override_settings
-from django.db import connection
+from django.db import connection, connections
 from django.db.migrations.loader import MigrationLoader, AmbiguityError
 from django.db.migrations.recorder import MigrationRecorder
 from django.utils import six
@@ -26,6 +26,12 @@ class RecorderTests(TestCase):
             recorder.applied_migrations(),
             set([("myapp", "0432_ponies")]),
         )
+        # That should not affect records of another database
+        recorder_other = MigrationRecorder(connections['other'])
+        self.assertEqual(
+            recorder_other.applied_migrations(),
+            set(),
+        )
         recorder.record_unapplied("myapp", "0432_ponies")
         self.assertEqual(
             recorder.applied_migrations(),