Querydsl：如何向下转换连接实体？

Question

I'm looking for a way to make a join on a one-to-many relation where the many-side is defined through inheritance and the right part of the join is restricted to a specific subclass (downcast).

Say I have the entities below (example taken from here ):

@Entity
public class Project {
  @Id
  @GeneratedValue
  private long id;
  private String name;
  @OneToMany(cascade = CascadeType.ALL)
  private List<Employee> employees;
    .............
}

@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@Entity
@DiscriminatorColumn(name = "EMP_TYPE")
public class Employee {
  @Id
  @GeneratedValue
  private long id;
  private String name;
    .............
}

@Entity
@DiscriminatorValue("F")
public class FullTimeEmployee extends Employee {
  private int annualSalary;
    .............
}

@Entity
@DiscriminatorValue("P")
public class PartTimeEmployee extends Employee {
  private int weeklySalary;
    .............
}

@Entity
@DiscriminatorValue("C")
public class ContractEmployee extends Employee {
  private int hourlyRate;
    .............
}

I can easily build join queries that involve properties defined in the superclass Employee , like:

JPAQuery query = ...
QProject project = new QProject("p");
QEmployee employee = new QEmployee("e");
query.join(project.employees, employee);
query.where(employee.name.startsWith("A"));

But if I want to access a property of subclass, say FullTimeEmployee.annualSalary , and hence restrict the join to that sub-type, how do I do that?

How do I build the the equivalent of following JPQL:

SELECT DISTINCT p FROM Project p JOIN TREAT(p.employees AS FullTimeEmployee) e WHERE e.annualSalary > 100000

Answer 1

You can do it like this:

EntityManager em = ...;
QProject p = QProject.project;
QFullTimeEmployee e = QFullTimeEmployee.fullTimeEmployee;
List<FullTimeEmployee> emps = new JPAQuery<>(em)
    .select(p)
    .distinct()
    .from(p)
    .innerJoin(p.employees, e._super)
    .where(e.annualSalary.gt(100000))
    .fetch();

See also this post on the Querydsl forum: https://groups.google.com/d/msg/querydsl/4G_ea_mQJgY/JKD5lRamAQAJ