使用Jersey的JAX-RS在休眠资源类中使用连接表进行多对一

Question

我正在使用Jersey实现RESTful Web服务。我使用hibernate模式与数据库（mySQL）通信。我的hibernate资源类包括：

@Entity
public class Activity {

    @Id
    @GeneratedValue
    private long id;

@ManyToOne
    @JoinTable(name="category_activity",
    joinColumns={@JoinColumn(name="activities_id")},
    inverseJoinColumns={@JoinColumn(name="Category_id")})
    private Category category;
}

和Category类：

@Entity
public class Category {

    @Id
    @GeneratedValue
    private long id;

    @OneToMany
    @Fetch(FetchMode.JOIN)
    @JoinTable(name = "category_activity",
    joinColumns = { @JoinColumn(name = "Category_id") }, 
    inverseJoinColumns = { @JoinColumn(name = "activities_id") })
    @JsonIgnore
    private Collection<Activity> activities;
}

我使用此查询来获取活动：

session.createQuery("from Activity a join a.category cs where cs.id= :categoryId order by a.key").setLong("categoryId", categoryId).list();

JSON格式的结果不正确，例如：

[[{"id":26,"key":"other","name":"Other","cost":100.0,"category":{"id":10,"name":"General","description":""}},{"id":10,"name":"General","description":""}]]

如您所见，类别被打印2次，并且在其周围有一个额外的[]。当我在Category类中使用一对多关系的另一种机制时：

@OneToMany(targetEntity = Activity.class, mappedBy = "category", fetch = FetchType.LAZY, cascade = CascadeType.ALL)
@JsonIgnore
private Collection<Project> activities;

在Activity类中：

@ManyToOne(optional = false)
    private Category category;

而这个查询：

session.createQuery("from Activity as a where a.category.id= :categoryId order by a.key").setLong("categoryId", categoryId).list();

一切正常。但是我必须使用联接表，因为我不打算更改数据库。

正确的结果应如下所示：

[{"id":26,"key":"other","name":"Other","cost":100.0,"category":{"id":10,"name":"General","description":""}}]

感谢您的帮助。

Answer 1

在多个方面定义联接表，但不要在一侧再次定义它。这将创建映射到同一表的两个单向关联，而不是一个双向关联。

双向关联始终具有所有者侧（您可以在其中指定要使用的连接列或连接表），以及通过使用mapledBy属性可以说是另一侧的反向的反向侧：

public class Activity {

    @ManyToOne // owner side: it doesn't have mappedBy, and can decide how the association is mapped: with a join table
    @JoinTable(name="category_activity",
               joinColumns={@JoinColumn(name="activities_id")},
               inverseJoinColumns={@JoinColumn(name="Category_id")})
    private Category category;
}

public class Category {
    @OneToMany(mappedBy = "category") // inverse side: it has a mappedBy attribute, and can't decide how the association is mapped, since the other side already decided it.
    @Fetch(FetchMode.JOIN)
    @JsonIgnore
    private Collection<Activity> activities;
}

编辑：

此外，您的查询应仅通过添加select子句来选择活动，而不是查询所连接的所有实体：

select a from Activity as a where a.category.id= :categoryId order by a.key