Switch parent back to a ForeignKeyField without a constraint

data/database.py

@@ -552,6 +552,9 @@ class UserRegion(BaseModel):
   )
 
 
+_ImageProxy = Proxy()
+
+
 class Image(BaseModel):
   # This class is intentionally denormalized. Even though images are supposed
   # to be globally unique we can't treat them as such for permissions and
@@ -574,7 +577,9 @@ class Image(BaseModel):
 
   security_indexed = BooleanField(default=False)
   security_indexed_engine = IntegerField(default=-1)
-  parent_id = IntegerField(index=True, null=True)
+
+  # We use a proxy here instead of 'self' in order to disable the foreign key constraint
+  parent = ForeignKeyField(_ImageProxy, index=True, null=True, related_name='children')
 
   class Meta:
     database = db
@@ -587,6 +592,9 @@ class Image(BaseModel):
     )
 
 
+_ImageProxy.initialize(Image)
+
+
 class RepositoryTag(BaseModel):
   name = CharField()
   image = ForeignKeyField(Image)