How To Check If A Transient Entity Exists In The Database Via Spring Query By Example (QBE)

Description: This application is an example of using Spring Data Query By Example (QBE) to check if a transient entity exists in the database. Consider the Book entity and a Spring controller that exposes an endpoint as: public String checkBook(@Validated @ModelAttribute Book book, ...). Beside writting an explicit JPQL, we can rely on Spring Data Query Builder mechanism or, even better, on Query By Example (QBE) API. In this context, QBE API is quite useful if the entity has a significant number of attributes and:

• for all attributes, we need a head-to-head comparison of each attribute value to the corresponding column value
• for a subset of attributes, we need a head-to-head comparison of each attribute value to the corresponding column value
• for a subset of attributes, we return true at first match between an attribute value and the corresponding column value
• any other scenario

Key points:

• the repository, BookRepository extends QueryByExampleExecutor
• the application uses <S extends T> boolean exists(Example<S> exmpl) with the proper probe (an entity instance populated with the desired fields values)
• moreover, the probe relies on ExampleMatcher which defines the details on how to match particular fields

Note: Do not conclude that Query By Example (QBE) defines only the exists() method. Check out all methods here.

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