java 如何分配多个字段作为实体的主键（使用 JPA）
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How to assign several fields as a primary key of an entity (using JPA)
提问by sheidaei
One can assign a primary key for its class by using @Id annotation in JPA. My question is what if one doesn't want to have an auto generated key in his tables and use fields (maybe more than one) as a primary key.
可以通过在 JPA 中使用 @Id 注释为其类分配主键。我的问题是，如果一个人不想在他的表中使用自动生成的键并使用字段（可能不止一个）作为主键，该怎么办。
Let's say we have a person table with SSN, NATIONALITY and NAME. SSN is defined as the number a person is identified by in his country. Thus, we might have two persons with the same number in two different countries. The primary key for this table can be SSN+NATIONALITY. Is there any way to map these two fields using JPA and map it to an object? or the only way it to create an auto generated id and use @Id annotation
假设我们有一个带有 SSN、NATIONALITY 和 NAME 的 person 表。SSN 被定义为一个人在他的国家被识别的号码。因此，我们可能在两个不同的国家有两个相同号码的人。此表的主键可以是 SSN+NATIONALITY。有没有办法使用JPA映射这两个字段并将其映射到对象？或创建自动生成的 id 并使用 @Id 注释的唯一方法
CREATE TABLE PERSON (
SSN INT,
NATIONALITY VARCHAR,
NAME VARCHAR
)
回答by michal
Yes, it is possible. A composite primary key consist of multiple primary key fields. Each primary key field must be one of the supported by JPA type. For your table, you have:
对的，这是可能的。复合主键由多个主键字段组成。每个主键字段必须是 JPA 类型支持的字段之一。对于您的表，您有：
@Entity @IdClass(PersonId.class)
public class Person {
@Id int ssn;
@Id String nationality;
....
}
For entity with multiple key, JPA requires defining a special ID class. This class should be attached to the entity class using the @IdClass annotation.
对于具有多个键的实体，JPA 需要定义一个特殊的 ID 类。此类应使用@IdClass 注释附加到实体类。
class PersonId {
int ssn;
String nationality;
}
The ID class reflects the primary key fields and its objects can represent primary key values
ID 类反映主键字段，其对象可以表示主键值
