强制 JPA 接受外键值，而不仅仅是实体

Question

I imported some JPA model classes from a MySQL schema.

Here is one of the sample classes that are auto generated by JPA:

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


/**
 * The persistent class for the surveys database table.
 * 
 */
@Entity
@Table(name="surveys")
@NamedQuery(name="Survey.findAll", query="SELECT s FROM Survey s")
public class Survey implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    @Column(unique=true, nullable=false)
    private int id;

    @Temporal(TemporalType.TIMESTAMP)
    @Column(name="done_at")
    private Date doneAt;

    //bi-directional many-to-one association to Form
    @ManyToOne
    @JoinColumn(name="form_id", nullable=false)
    private Form form;

    //bi-directional many-to-one association to User
    @ManyToOne
    @JoinColumn(name="user_id", nullable=false)
    private User user;

    public Survey() {
    }

    public int getId() {
        return this.id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public Date getDoneAt() {
        return this.doneAt;
    }

    public void setDoneAt(Date doneAt) {
        this.doneAt = doneAt;
    }

    public Form getForm() {
        return this.form;
    }

    public void setForm(Form form) {
        this.form = form;
    }

    public User getUser() {
        return this.user;
    }

    public void setUser(User user) {
        this.user = user;
    }

}

The problem of this JPA model is, it just accepts entities for the foreign keys. I mean here I have form_id and user_id as the foreign keys: However I don't have any set functions like the following:

public void setForm(int formId) {
    Form form = find from table forms with id equal to formId; 
    this.form = form;
}

or 

public void setForm(int formId) {
    this.formId = formId;
}

How can I force JPA to generate such kind of functions also when it tries to import the schema??

If it is not possible what are the best practices for extending the JPA model class? I mean as it may happen in the future by changing of the database schema I need to import the JPA models again, maybe it is not a good idea to implement these functions by hand inside the JPA model and manipulate it by hand.

Maybe you ask why do I need such kinds of set functions that accept ids instead of entities? My program reads some mockdata and tries to fill the tables in the MySQL. The problem is in the mock data I have foreign key of the User and Form entities. They are already exist in the DB, and I don't need to create them again.

Answer 1

You cannot use FK-s in JPA as is. You can use JPQL/SQL to update the given row's field with the FK or you can create a service class, where you load the Form by id and then set it to the given Survey. You should avoid to extend an auto generated class (and its agains Open-Close principle too).

Edit: Create a class, for example FormService. There you can create a method and use Spring Data JPA, JpaTemplate, Plain JPA or other methods to retrieve the Form. For example, with hibernate:

Form form = session.load(Form.class, id);

Then you set this to the Survey.

survay.setForm(form);

And now just save it back (persist if it is a new survey or merge if not).

session.merge(survay);

If you use Spring, it may easier and better to use Data Jpa or JpaTemplate, but the previous one will work with Hibernate (and other JPA implementations) in any java environment.