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hibernatejpafetching-strategy

Hibernate ignores FetchType for Single Inheritance

I have 3 entities inheriting from another entity. I'm using the strategy Single_Table

@Inheritance(strategy = InheritanceType.SINGLE_TABLE)

E.g. Class B,C and D inherite from A

On entity B I'm loading another entity X eagerly. Unfortunately, Hibernate ignores my annotation and creates a select for each entity B to fetch entity X.

@ManyToOne(fetch=FetchType.EAGER)
private Projekt projekt;

My select statement looks as follows: ´select a from A a´

Some more code examples:

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "DType", discriminatorType = DiscriminatorType.STRING, length = 3)
public abstract class A {}


@Entity
@DiscriminatorValue(B.PREFIX)
public class B extends A {

   @ManyToOne(fetch=FetchType.EAGER)
   private Projekt projekt;
}

Now I expect Hibernate to query all classes which inherite from A, which it does. Unfortunately it also executes a select statement for each result row to query "projekt", which I want to avoid.

Solution

  • Hibernate doesn't insert joins into SQL queries generated from the HQL/JPQL ones for FetchType.EAGER.

    In order to insert join you need to specify left join fetch explicitly, I guess it won't create problems for InheritanceType.SINGLE_TABLE despite the fact that this relationship doesn't exists in other subclasses:

    from A a left join fetch a.projekt