How to extract data from two GQL classes?

I have the following 2 classes:

class UsersRSS(db.Model):
    userId = db.IntegerProperty()
    fileHash = db.StringProperty()
    ad开发者_JS百科ded = db.DateTimeProperty(auto_now_add=True)

class Files(db.Model):
    fileHash = db.StringProperty()
    title = db.StringProperty()
    file = db.BlobProperty()
    added = db.DateTimeProperty(auto_now_add=True)    

I need to create a GQL query, which will return a data from both tables:

items = UsersRSS.gql("WHERE userId = :userId ORDER BY date DESC LIMIT 10", userId=user_id)

But each item besides fileHash and added fields, should contain fields from the second class - title and file (matching should be done by fileHash field).

There's no way to retrieve from Datastore different Kinds using the same GQL Select statement.

Another option would be using the ReferenceProperty to create a many-to-one relationship between UsersRSS and Files, refactoring your code in this way:

class UsersRSS(db.Model):
    userId = db.IntegerProperty()
    file = db.ReferenceProperty(Files)
    added = db.DateTimeProperty(auto_now_add=True)

class Files(db.Model):
    fileHash = db.StringProperty()
    title = db.StringProperty()
    file = db.BlobProperty()
    added = db.DateTimeProperty(auto_now_add=True)

items = UsersRSS.gql("WHERE userId = :userId ORDER BY date DESC LIMIT 10", userId=user_id)

In this way, the Files properties will be automatically dereferenced using the item.file.title dot notation:

for item in items:
    print item.UserID
    print item.Added
    print item.file.title #This costs an additional RPC call to Datastore

To save your App from the ReferenceProperty RPC overhead have a look to the evergreen article ReferenceProperty prefetching.