一对多关系无需使用"distinct"即可获得重复的对象.为什么?

问题描述

我有2个一对多关系的类和一个有点奇怪的HQL查询.即使我已经阅读了一些已发布的问题，对我来说也不清楚.

I have 2 classes in a one-to-many relationship and a HQL query that is a bit strange. Even if I have read some questions already posted, it does not seem clear to me.

Class Department{
   @OneToMany(fetch=FetchType.EAGER, mappedBy="department")
   Set<Employee> employees;
}
Class Employee{
   @ManyToOne
   @JoinColumn(name="id_department")
   Department department;
}

当我使用以下查询时，我得到重复的Department对象:

When I use the following query I get duplicates Department objects:

session.createQuery("select dep from Department as dep left join dep.employees");

因此，我必须使用与众不同的内容:

Thus, I have to use distinct:

session.createQuery("select distinct dep from Department as dep left join dep.employees");

此行为是预期的吗?与SQL进行比较，我认为这很不正常.

Is this behaviour an expected one? I consider this unusual, as comparing it with SQL.

推荐答案

此问题在:

List result = session.createCriteria(Order.class)
                        .setFetchMode("lineItems", FetchMode.JOIN)
                        .list();

<class name="Order">
    <set name="lineItems" fetch="join">
    ...
</class>

List result = session.createCriteria(Order.class)
                        .list();

---

List result = session.createQuery("select o from Order o left join fetch o.lineItems").list();

SELECT o.*, l.* from ORDER o LEFT OUTER JOIN LINE_ITEMS l ON o.ID = l.ORDER_ID

Collection result = new LinkedHashSet( session.create*(...).list() );

LinkedHashSet过滤掉重复的引用(这是一个集合)，然后 它保留插入顺序(结果中元素的顺序).那 太容易了，所以您可以在许多不同且更困难的地方完成 方式:

A LinkedHashSet filteres out duplicate references (it's a set) and it preserves insertion order (order of elements in your result). That was too easy, so you can do it in many different and more difficult ways:

List result = session.createCriteria(Order.class)
                        .setFetchMode("lineItems", FetchMode.JOIN)
                        .setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY)
                        .list();


<class name="Order">
    ...
    <set name="lineItems" fetch="join">

List result = session.createCriteria(Order.class)
                        .setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY)
                        .list();

List result = session.createQuery("select o from Order o left join fetch o.lineItems")
                      .setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY) // Yes, really!
                      .list();

List result = session.createQuery("select distinct o from Order o left join fetch o.lineItems").list();

这篇关于一对多关系无需使用"distinct"即可获得重复的对象.为什么?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！