How To Handle Entities Inheritance With Spring Data Repositories

Description: Consider the JPA Single Table inheritance strategy (JOINED). It can be any other JPA inheritance (e.g., SINGLE_TABLE), but let's use this one here. And, the following entity hierarchy:

For these three entities, Book, Paperback and Ebook, we have the corresponding repositories, BookRepository, PaperbackRepository and EbookRepository. But, if we write a query-method as findByTitle() we should duplicate it in each of these repositories in order to call BookRepository#findByTitle(), PaperbackRepository#findByTitle() and EBookRepository#findByTitle(). But, we know that Paperback and Ebook are actually subclasses of Book, therefore they inherit the Book class. It will be useful to do the same thing for our repositories. It will be better to write the findByTitle() query-method only once and use it in all these repositories instead of duplicating it in each repository. This application shows you how to do it.

Key points:

• define the findByTitle() in a @NoRepositoryBean class (let's name this class, BookBaseRepository)
• next, BookRepository, PaperbackRepository and EbookRepository extends BookBaseRepository
• for queries express via @Query use #{#entityName} and Spring will replace it with the proper entity name

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