Converting JPA specification to JPQL

Question

we have JPA Specifications defined on an entity. I want to convert that specification to JPQL So I can create join it with some other entity. (I tried using parent-child relationship but that's not efficient, and that doesn't allow us to put filtering(specifications) on chile entity). Earlier We were reading from a single table so we had specifications in place that was working fine. Now we need to join this with another entity, For that I want to use JPQL over specifications. I want to convert specification on one table to JPQL and than join it with other entity.

Answer 1

Use @query annotation on your repository method and write your JPQL query inside the @query annotation.

you can refer table names with your entity name inside the @query annotation.

Answer 2

Let say you have User and List entities

In JPQL you will write something like this

public List<User> getUserByName(String username) {
    String queryString = "select u from User u where u.name= '"+username+"' ";
    Query query = getEntityManager().createQuery(queryString);
    return query.getResultList();
}

With JPA and Hibernate older than 5.1

even with JPA 2.1 and Hibernate 5.0 it’s still not possible to join two unrelated entities.

If User and UserAddress are not related to each other

List<Object[]> results = em.createQuery("SELECT u FROM User u, UserAddress ua WHERE u.firstName = ua.firstName AND u.lastName = ua.lastName").getResultList();

Hibernate will transform this as cross join something like this

User user01_ cross join UserAddress useraddress1_

With Hibernate 5.1

Same quesry will be treated as inner Join

List<Object[]> results = em.createQuery("SELECT u FROM User u, UserAddress ua WHERE u.firstName = ua.firstName AND u.lastName = ua.lastName").getResultList();

User user01_ inner Join UserAddress useraddress1_

Other Imp Point:

Using subqueries in the join clause is not allowed. in JPQL, subqueries are only allowed in the WHERE and HAVING part.