port label support to refactored v2 registry

data/database.py

@@ -957,7 +957,7 @@ class ServiceKey(BaseModel):
   rotation_duration = IntegerField(null=True)
   approval = ForeignKeyField(ServiceKeyApproval, null=True)
 
-'''
+
 class MediaType(BaseModel):
   """ MediaType is an enumeration of the possible formats of various objects in the data model. """
   name = CharField(index=True, unique=True)
@@ -992,6 +992,21 @@ class TagManifestLabel(BaseModel):
       (('annotated', 'label'), True),
     )
 
+
+'''
+
+class ManifestLabel(BaseModel):
+  repository = ForeignKeyField(Repository, index=True)
+  annotated = ForeignKeyField(Manifest, index=True)
+  label = ForeignKeyField(Label)
+
+  class Meta:
+    database = db
+    read_slaves = (read_slave,)
+    indexes = (
+      (('repository', 'annotated', 'label'), True),
+    )
+
 class Blob(BaseModel):
   """ Blob represents a content-addressable object stored outside of the database. """
   digest = CharField(index=True, unique=True)