Hibernate：自我加入混乱？

Question

我有一个category 表。其中前 5 个是主要类别，并且 其他是子类别。

我需要获取前 5 个主要类别的子类别，所以我找到了 sql 查询

SELECT m.category_id,m.category_name  AS 'Parent',
      e.category_name AS 'Sub'
FROM category e
INNER JOIN category m ON m.category_id = e.parent_category_id
ORDER BY Parent

查询正在加入同一个表本身。并得到下面给出的结果

结果

如何将 SQL 查询转换为 HQL 并将如上图所示的数据返回给用户 标准的json格式？

FetchSubCategory

import java.io.Serializable;
import java.util.Set;
import javax.persistence.*;

@Entity
@Table(name = "category")
public class FetchSubCategory implements Serializable {

    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "category_id")
    private Integer categoryId;

    @Column(name = "category_name")
    private String categoryName;

    @ManyToOne(cascade = {CascadeType.ALL})
    @JoinColumn(name = "parent_category_id")
    private FetchSubCategory parent;

    @OneToMany(mappedBy = "parent")
    private Set<FetchSubCategory> subCategory;

    public Integer getCategoryId() {
        return categoryId;
    }

    public void setCategoryId(Integer categoryId) {
        this.categoryId = categoryId;
    }

    public String getCategoryName() {
        return categoryName;
    }

    public void setCategoryName(String categoryName) {
        this.categoryName = categoryName;
    }

    public FetchSubCategory getParent() {
        return parent;
    }

    public void setParent(FetchSubCategory parent) {
        this.parent = parent;
    }

    public Set<FetchSubCategory> getSubCategory() {
        return subCategory;
    }

    public void setSubCategory(Set<FetchSubCategory> subCategory) {
        this.subCategory = subCategory;
    }

}

方法

public Set<FetchSubCategory> fetchSubCategory() throws SQLException, ClassNotFoundException, IOException {
        Set<FetchSubCategory> groupList = null;
        try {
            Session session = sessionFactory.getCurrentSession();
            Query query = session.createQuery("SELECT m.categoryName AS 'Parent', e.categoryName AS 'Sub' FROM FetchSubCategory e INNER JOIN FetchSubCategory m ORDER BY Parent");

            groupList = (Set<FetchSubCategory>) query.list();
        } catch (Exception e) {
            e.printStackTrace();
        }

        return groupList;
    }

谁能纠正我的错误并告诉我如何获取如上图所示的结果？

Answer 1

对于您的情况下的自我加入，以下内容适合您。

    @ManyToOne(cascade={CascadeType.ALL})    
    @JoinColumn(name = "parent_category_id")
    private FetchSubCategory parent;

    @OneToMany(mappedBy = "parent")
    private Set<FetchSubCategory> subCategory;

FetchSubCategory 实体类，我们定义了两个属性：FetchSubCategory parent and Set<FetchSubCategory> subCategory。属性 parent 用@ManyToOne 注解映射，从属属性用@OneToMany 映射。同样在@OneToMany 属性中，我们将mappedBy="parent" 定义为relationship owner，从而管理表中的外部关系。

注释@JoinColumn 也定义在parent 上，使其成为关系所有者。 @JoinColumn 定义了连接列，在我们的例子中是parent_category_id。

Answer 2

这些东西会解决你的问题

@Entity
@Table(name = "category")
public class FetchSubCategory implements Serializable {

    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "category_id")
    private Integer categoryId;

    @Column(name = "category_name")
    private String categoryName;

    @NotFound(action = NotFoundAction.IGNORE)
    @ManyToOne
    @JsonIgnore
    @JoinColumn(name = "parent_category_id")
    private FetchSubCategory mainCategory;

    @JsonSerialize(include = JsonSerialize.Inclusion.NON_EMPTY)//Avoiding empty json arrays.objects
    @OneToMany(mappedBy = "mainCategory", fetch = FetchType.EAGER)
    private List<FetchSubCategory> subCategory;

    public Integer getCategoryId() {
        return categoryId;
    }

    public void setCategoryId(Integer categoryId) {
        this.categoryId = categoryId;
    }

    public String getCategoryName() {
        return categoryName;
    }

    public void setCategoryName(String categoryName) {
        this.categoryName = categoryName;
    }

    public FetchSubCategory getMainCategory() {
        return mainCategory;
    }

    public void setMainCategory(FetchSubCategory mainCategory) {
        this.mainCategory = mainCategory;
    }

    public List<FetchSubCategory> getSubCategory() {
        return subCategory;
    }

    public void setSubCategory(List<FetchSubCategory> subCategory) {
        this.subCategory = subCategory;
    }

获取您的子类别

public List<FetchSubCategory> fetchSubCategory() throws SQLException, ClassNotFoundException, IOException {
        List<FetchSubCategory> groupList = null;
        try {
            Session session = sessionFactory.getCurrentSession();
            Query query = session.createQuery("select distinct e FROM FetchSubCategory e INNER JOIN e.subCategory m ORDER BY m.mainCategory");
            groupList = query.list();
        } catch (Exception e) {
            e.printStackTrace();
        }

        return groupList;
    }