java - 为什么 Hibernate 不为多对多关系表生成主键？

Question

我有一个简单的多对多表。一个 Term 可以属于多个 Categories，一个 Category 可以分配给多个 Term。

术语.java

@Entity
public class Term implements Serializable {

    @Id
    @GeneratedValue
    private long id;

    @NotNull
    @Size(min = 1, max = 100)
    private String name;

    @ManyToMany(mappedBy="terms")
    private List<Category> categories = new ArrayList<Category>();

    ...
    // getters and setters
}

分类.java

@Entity
public class Category implements Serializable {

    @Id
    @GeneratedValue
    private long id;

    @NotNull
    @Size(min = 1, max = 255)
    private String name;

    @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    private List<Term> terms = new ArrayList<>();

   // getters and setters
}

然而，生成的交集表“”不包含主键，从这里可以看出:

MariaDB [wordDS]>describe Category_Term;
+---------------+------------+------+-----+---------+-------+
| Field         | Type       | Null | Key | Default | Extra |
+---------------+------------+------+-----+---------+-------+
| categories_id | bigint(20) | NO   | MUL | NULL    |       |
| terms_id      | bigint(20) | NO   | MUL | NULL    |       |
+---------------+------------+------+-----+---------+-------+

复合键不应该总是需要作为主键，并且应该由 Hibernate 自动创建吗？我怎样才能自动创建它？

Answer 1

使用 Set 而不是 List 来强制唯一性。 Hibernate 将生成主键。

@ManyToMany(mappedBy="terms")
private Set<Category> categories = new HashSet<Category>();

/* ... */

@ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
private Set<Term> terms = new HashSet<>();