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class Poi(GeoPoint):    MAX_GEOCELL_RESOLUTION = 8    poi_type = db.StringProperty()    poi_name = db.StringProperty()class Spot(GeoPoint):    like_count = db.IntegerProperty()    dislike_count = db.IntegerProperty()    poi = db.ReferenceProperty(Poi)class Rating(GeoPoint):    like = db.BooleanProperty()    spot = db.ReferenceProperty(Spot)    user = db.ReferenceProperty(User)    creation_date = db.DateTimeProperty(auto_now_add=True)