The first thing many developers try to do when they begin using Hibernate or Spring Data JPA is to map a parent/child relationship. This is usually the first time they encounter collections. It’s also the first time they have to think about the differences between entities and value types or get lost in the complexity of ORM.
Managing the associations between classes and the relationships between tables is at the heart of ORM. Most of the difficult problems involved in implementing an ORM solution relate to collections and entity association management. We’ll start this chapter with some basic collection-mapping concepts and simple examples. After that, you’ll be prepared for the first collection in an entity association—we’ll come back to more complicated entity association mappings in the next chapter. To get the full picture, we recommend you read both this chapter and the next.