使用 JPA 标准 API 的 JOIN 查询中的反向类型

Question

I would like to return a list of Group objects using a JPA criteria query. A group can either be empty or contain participants . Each Participant has a reference to its Group . The group knows nothing about the participants. Now I would like to implement a criteria query to dynamically return either:

1. Only groups with participants.
2. All groups (including empty ones).

Basically, this is my Group class:

public class Group {
  private long id;
}

And this is my Participant :

public class Participant {
  @ManyToOne
  @JoinColumn(name = "FK_GROUP")
  private Group group;
}

I left out all properties, methods and annotations for sake of simplicity.

This will return both, empty groups and groups with participants, in plain SQL:

SELECT DISTINCT G.* FROM GROUP_TABLE G
  LEFT JOIN PARTICIPANT_TABLE P ON P.FK_GROUP=G.ID;

Now I'd like to do the same thing with Criteria API:

if (includeEmptyGroups) {
  // ...
  Root<Group> root = criteria.from(Group.class);
  Join<Participant, Group> join = root.join("id"); <-- This is my issue!
  criteria.select(root).distinct(true);
  //...
}
else {
  // ...
}

The query should return a list of Group objects, so I chose that type for my Root object and the from() method. Now, most tutorials would define a Join<Group, Participant> , and join participants by a property of the Group class.

Due to my reversed class structure, however, I cannot do this. The code above does not work, because I reversed the types and JPA cannot join by simple types. I'm unsure, if I approach the problem correctly at all.

I could switch types around and base my Root on the Participant class, then join with Group . This, however, makes it impossible to conditionally return empty groups. Is it even possible to build the plain SQL query with Criteria API?

Answer 1

Since the JPA contracts do not allow for left joining entities, you can only use HQL or a query builder API on top of HQL like eg Blaze-Persistence which supports this. Either way, you will need at least Hibernate 5.1 which is the first version that added support for entity joins.