This User model is used throughout the bundle but doesn’t have a complete implementation to access its own data (I didn’t look but I think it could be an interface only). The final implementation is done in an entity class and another in a document class so no matter whether you use MySQL or MongoDB ...
My entities are Series, which has many Seasons, which have many Episodes. When an episode is created or updated I want to also update its related Series so that it always has correct firstYear and lastYear data for when the series was broadcasted.
This chapter introduces association mappings which are used to explain references between objects and are mapped to a relational database using foreign keys.