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

I don't know what Spring-Data is doing there, but to do that, you usually have to use the TREAT operator to be able to access the sub-association but the implementation for that Operator is quite buggy. Hibernate supports implicit subtype property access which is what you would need here, but apparently Spring-Data can't handle this properly. I can recommend that you take a look at Blaze-Persistence Entity-Views, a library that works on top of JPA which allows you map arbitrary structures against your entity model. You can map your DTO model in a type safe way, also the inheritance structure. Entity views for your use case could look like this

@EntityView(Answer.class)
interface AnswerDTO {
  @IdMapping
  Long getId();
  ValueDTO getValue();
}
@EntityView(Value.class)
@EntityViewInheritance
interface ValueDTO {
  @IdMapping
  Long getId();
}
@EntityView(TextValue.class)
interface TextValueDTO extends ValueDTO {
  String getText();
}
@EntityView(RatingValue.class)
interface RatingValueDTO extends ValueDTO {
  int getRating();
}
@EntityView(MCValue.class)
interface TextValueDTO extends ValueDTO {
  @Mapping("selected.id")
  Set getOption();
}

With the spring data integration provided by Blaze-Persistence you can define a repository like this and directly use the result

@Transactional(readOnly = true)
interface AnswerRepository extends Repository {
  List findAll();
}

It will generate a HQL query that selects just what you mapped in the AnswerDTO which is something like the following.

SELECT
  a.id, 
  v.id,
  TYPE(v), 
  CASE WHEN TYPE(v) = TextValue THEN v.text END,
  CASE WHEN TYPE(v) = RatingValue THEN v.rating END,
  CASE WHEN TYPE(v) = MCValue THEN s.id END
FROM Answer a
LEFT JOIN a.value v
LEFT JOIN v.selected s