How does Hibernate handle union primary keys?

Hibernate uses the @IdClass annotation to process the joint primary key. The joint primary key consists of multiple columns, and each entity corresponds to a joint primary key class. They are used for uniqueness in related tables and provide greater query and update flexibility. For example, in a user management system, a user is uniquely identified by firstName and lastName. The joint primary key UserId contains these two attributes and is obtained through the get method. User for a specific primary key.

Hibernate Handling Union Primary Keys

A union primary key is a primary key that uses multiple columns to uniquely identify rows in a database table. Hibernate handles union primary keys by using the @IdClass annotation.

Code Example:

Suppose we have a table named User with firstName and lastName The columns together form the joint primary key. We can use the following entity classes to map this table:

@Entity
public class User {

    @IdClass(UserId.class)
    private UserId id;
    
    // 其他属性和方法（省略）
    
}

public class UserId implements Serializable {

    private String firstName;
    private String lastName;

    // 构造函数、getter 和 setter 方法（省略）

}

In this example, the UserId class represents the union primary key, which contains firstName and lastName Attributes.

Practical case:

We use joint primary keys in a user management system. Each user is uniquely identified by their firstName and lastName.

We use the following code to find the user with a specific union primary key from the database:

UserId id = new UserId("John", "Doe");
User user = session.get(User.class, id);

In the above code, the get method will return the user with the union primary key User instances for John and Doe.

Union primary keys are useful for enforcing uniqueness between related tables. They also allow multiple columns to be used to identify entities, providing greater flexibility when querying and updating.

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