Multiple relationships between two entities

When two entities have more than one relationship between them, Pony requires the reverse attributes to be specified. This is needed in order to let Pony know which pair of attributes are related to each other. Let’s consider the data diagram where a user can write tweets and also can favorite them:

class User(db.Entity):
    tweets = Set("Tweet", reverse="author")
    favorites = Set("Tweet", reverse="favorited")
class Tweet(db.Entity):
    author = Required(User, reverse="tweets")
    favorited = Set(User, reverse="favorites")

In the example above we have to specify the option reverse. If you will try to generate mapping for the entities definition without the reverse specified, you will get the exception pony.orm.core.ERDiagramError: "Ambiguous reverse attribute for Tweet.author". That happens because in this case the attribute author can technically relate either to the attribute tweets or to favorites and Pony has no information on which one to use.