Welcome to OGeek Q&A Community for programmer and developer-Open, Learning and Share
Welcome To Ask or Share your Answers For Others

Categories

0 votes
1.0k views
in Technique[技术] by (71.8m points)

java - How to join unrelated entities with the JPA Criteria API

Two database tables have a foreign key relationship.

They are mapped to two entities A and B by JPA, but the join columns are manually removed from the entities, so in JPA world classes A and B are not related and you cannot navigate from one to the other through a field/property.

Using the JPA Criteria API, is it possible to create a query which joins the two tables?

All examples I found on internet uses the join column to achieve the goal, but, as stated above, it was removed from the code because most time I'm not interested in the relationship between A and B and I'm afraid about possible overhead.

See Question&Answers more detail:os

与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
Welcome To Ask or Share your Answers For Others

1 Reply

0 votes
by (71.8m points)

First: Foreign key relationship are not only for navigating. They mainly serve to ensure that no spurious values are introduced in the relationship. They also may help the database for query optimization. I would advise you to reconsider that.

Anyway, for creating a query that uses several unrelated entities, you need to put them as from (root) entities (as you would do in SQL or JPQL)

SELECT .... FROM Link l, Training t WHERE l.attribute = t.attribute;

Root<Link> rootLink = criteriaQuery.from(Link.class);
Root<Training> rootTraining = criteriaQuery.from(Training.class);
...
criteriaQuery.where(
    criteriaBuilder.equal(rootLink.get(link_.linkAttribute), trainingLink));

与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
OGeek|极客中国-欢迎来到极客的世界,一个免费开放的程序员编程交流平台!开放,进步,分享!让技术改变生活,让极客改变未来! Welcome to OGeek Q&A Community for programmer and developer-Open, Learning and Share
Click Here to Ask a Question

...