JPA inheritance @EntityGraph include optional associations of subclasses

Question

Given the following domain model, I want to load all Answers including their Values and their respective sub-children and put it in an AnswerDTO<

Answer 1

You can only use an EntityGraph if the association attribute is part of the superclass and by that also part of all subclasses. Otherwise, the EntityGraph will always fail with the Exception that you currently get.

The best way to avoid your N+1 select issue is to split your query into 2 queries:

The 1st query fetches the MCValue entities using an EntityGraph to fetch the association mapped by the selected attribute. After that query, these entities are then stored in Hibernate's 1st level cache / the persistence context. Hibernate will use them when it processes the result of the 2nd query.

@Query("SELECT m FROM MCValue m") // add WHERE clause as needed ...
@EntityGraph(attributePaths = {"selected"})
public List findAll();

The 2nd query then fetches the Answer entity and uses an EntityGraph to also fetch the associated Value entities. For each Value entity, Hibernate will instantiate the specific subclass and check if the 1st level cache already contains an object for that class and primary key combination. If that's the case, Hibernate uses the object from the 1st level cache instead of the data returned by the query.

@Query("SELECT a FROM Answer a")
@EntityGraph(attributePaths = {"value"})
public List findAll();

Because we already fetched all MCValue entities with the associated selected entities, we now get Answer entities with an initialized value association. And if the association contains an MCValue entity, its selected association will also be initialized.