JPA:如何热切获取嵌入式元素集合 [英] JPA: How to fetch eagerly an embedded element collection

问题描述

考虑以下模型

@Entity
// JPA and JAXB annotations here
public class Employee implements Serializable {
     // other fields, annotations, stuffs
     ...
     @ElementCollection(fetch = FetchType.LAZY,
        targetClass = Address.class)
     @CollectionTable(name = "employee_address",
        schema = "hris",
        joinColumns = @JoinColumn(name = "employee_id",
                nullable = false,
                referencedColumnName = "employee_id",
                foreignKey = @ForeignKey(ConstraintMode.CONSTRAINT)))
     protected Set<Address> addresses;
     // setters, getters
     ...
 }

 @Embeddable
 // JAXB annotations here
 public class Address implements Serializable {
      // fields, setters, getters
 }

Address类带有@Embeddable注释，而Employee类具有addresses的嵌入式元素集合.元素集合的fetch设置为FetchType.LAZY.现在，我想创建一个@NamedQuery，该地址将检索所有具有预先初始化地址的雇员.知道JOIN FETCH 仅适用于带有 >或@ManyToMany基于JPA 2.1，我将如何创建有效的JPQL查询，以使我能够急切地检索嵌入式元素集合?

The Address class is annotated with @Embeddable annotation, and the Employee class has an embedded element collection of addresses. The element collection's fetch is set to FetchType.LAZY. Now, I would like to create a @NamedQuery that would retrieve all employees with addresses eagerly initialized. Knowing that JOIN FETCH will only work with entity collections annotated with @OneToMany or @ManyToMany based on JPA 2.1, how would I create a valid JPQL query that would allow me to eagerly retrieve embedded element collections?

推荐答案

在JPA 2.1规范(JSR 338)中，我找不到任何提示，表明访存联接仅适用于实体关系(但不适用于可嵌入对象). JSR 338第4.4.5.3节甚至规定:

In the JPA 2.1 specification (JSR 338) I cannot find any hint that fetch joins only work on entity relationships (but not embeddables). JSR 338, section 4.4.5.3 even states:

FETCH JOIN允许获取关联或元素集合，作为执行查询的副作用.

A FETCH JOIN enables the fetching of an association or element collection as a side effect of the execution of a query.

作为另一个提示，以下是作为JPA提供程序使用Hibernate 4.3.11执行的最小示例(基本上类似于您的示例)会导致单个查询:

As another hint the following minimal example (essentially resembling yours) executed with Hibernate 4.3.11 as JPA provider results in a single query:

可嵌入地址:

@Embeddable public class Address { private String city; }

员工实体:

@Entity public class Employee {

    @Id private Long id;

    @ElementCollection(fetch = FetchType.LAZY)
    @CollectionTable(name = "address", 
           joinColumns = @JoinColumn(name="employee_id"))
    private Set<Address> addresses;

}

JPQL查询:

em.createQuery("select e from Employee e join fetch e.addresses").getResultList();

产生的SQL查询:

select
    employee0_.id as id1_1_,
    addresses1_.employee_id as employee1_1_0__,
    addresses1_.city as city2_5_0__ 
from
    Employee employee0_ 
inner join
    address addresses1_ on employee0_.id=addresses1_.employee_id

因此上述JPQL查询似乎可以解决您的问题.

So the above JPQL query seems to solve your problem.

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