使用hibernate / spring-boot连接3个表

Question

我在理解如何使用spring-boot / hibernate连接3个表时遇到问题。表包括:用户、技术、类别

每个用户都有所有的10个类别，但在这些类别中，他们可以节省一项或多项技术。每种技术都可以在几个不同的类别中列出。

我有一个部分工作的代码，而不是引用表类别，我只是创建新的类别，所以我在我的BDD中有重复。理想情况下，我希望每个用户的数据结构都像这样(在伪代码中)：

{
    {
    "category1" : {id, name}
    "technologies" [{id, name}, {id, name}, {id, name} ]
    },

    {
    "category2" : {id, name}
    "technologies

    }
    .
    .
    .   
}

我的表格是：

用户表

public class MyUser {

// other properties

@OneToMany(mappedBy="id")
    private Collection<Category> categories;
}

技术

public class Technology {

// other properties

    @Id
    @SequenceGenerator(name = "id_seq", sequenceName = "id_seq", allocationSize = 1, initialValue = 1)
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "id_seq")
    private int id;

    @Column(name = "name")
    private String name;
}

技术类别：

public class TechnologyCategory {

// other properties

    @Id
    @SequenceGenerator(name = "id_seq", sequenceName = "id_seq", allocationSize = 1, initialValue = 1)
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "id_seq")
    private int id;

    @Column(name="name")
    private String name;
}

以及我试图将用户与类别联系起来的表格(每个类别都有技术列表)

USER_CATEGORIES

public class UserCategory {

    @Id
    @SequenceGenerator(name = "id_seq", sequenceName = "id_seq", allocationSize = 1, initialValue = 1)
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "id_seq")
    private int id;

    @Column(name = "name")
    private String technologyCategory; // here I would love to reference technology category table

    @ManyToMany()
    Collection <Technology> technologies;
}

因此，我已经尝试/阅读了以下内容：Joining three tables using MySQL

ManyToManyToMany - Joining three tables with Hibernate annotations

Hibernate: How to Join three 3 tables in one join table in Annotation?

Hibernate: mapping 3 tables

但是没有成功，因为每次尝试实现上面的解决方案都会导致异常(都与hibernate连接，无法创建表)，这是我无法解决的。谢谢

Answer 1

如果我没有理解错的话，假设你的示例代码可以工作，那么下面的代码就可以为你工作了：

User类，它加入了以下类别：

public class User {

    @Id
    @SequenceGenerator(name = "id_seq", sequenceName = "id_seq", allocationSize = 1, initialValue = 1)
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "id_seq")
    private int id;

    // other properties

    @ManyToMany(fetch = FetchType.EAGER)
    @JoinTable(name = "user_categories", joinColumns = @JoinColumn(name = "user_id", referencedColumnName = "id"),
            inverseJoinColumns = @JoinColumn(name = "category_id", referencedColumnName = "id"))
    private Set<Category> categories;
    
}

类别实体，其中包含技术的类别和技术：

public class Category {

    @Id
    @SequenceGenerator(name = "id_seq", sequenceName = "id_seq", allocationSize = 1, initialValue = 1)
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "id_seq")
    private int id;

    // other properties

    @ManyToOne
    @JoinColumn(name = "technology_category_id")
    private TechnologyCategory category;
    
    @ManyToMany(fetch = FetchType.EAGER)
    @JoinTable(name = "category_technologies", joinColumns = @JoinColumn(name = "category_id", referencedColumnName = "id"),
            inverseJoinColumns = @JoinColumn(name = "technology_id", referencedColumnName = "id"))
    private Set<Technology> technologies;
    
}

TechnologyCategory实体：

public class TechnologyCategory {

    // other properties

    @Id
    @SequenceGenerator(name = "id_seq", sequenceName = "id_seq", allocationSize = 1, initialValue = 1)
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "id_seq")
    private int id;

    @Column(name="name")
    private String name;
}

技术实体：

public class Technology {

    // other properties

    @Id
    @SequenceGenerator(name = "id_seq", sequenceName = "id_seq", allocationSize = 1, initialValue = 1)
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "id_seq")
    private int id;

    @Column(name="name")
    private String name;
}

如果不希望id出现在JSON中，只需将@JsonIgnore注释放在id属性之上即可。