How can I mark a foreign key constraint using Hibernate annotations?

Question

I am trying to use Hibernate annotation for writing a model class for my database tables.

I have two tables, each having a primary key User and Question.

@Entity @Table(name="USER") public class User {     @Id     @Column(name="user_id")     @GeneratedValue(strategy=GenerationType.AUTO)     private Long id;      @Column(name="username")     private String username;      // Getter and setter } 

Question Table.

@Entity @Table(name="QUESTION") public class Questions extends BaseEntity{      @Id     @Column(name="question_id")     @GeneratedValue(strategy=GenerationType.AUTO)     private int id;      @Column(name="question_text")     private String question_text;      // Getter and setter } 

And I have one more table, UserAnswer, which has userId and questionId as foreign keys from the above two tables.

But I am unable to find how I can reference these constraints in the UserAnswer table.

@Entity @Table(name="UserAnswer ") public class UserAnswer {     @Column(name="user_id")     private User user;      //@ManyToMany     @Column(name="question_id")     private Questions questions ;      @Column(name="response")     private String response;      // Getter and setter } 

How can I achieve this?

Answer 1

@Column is not the appropriate annotation. You don't want to store a whole User or Question in a column. You want to create an association between the entities. Start by renaming Questions to Question, since an instance represents a single question, and not several ones. Then create the association:

@Entity @Table(name = "UserAnswer") public class UserAnswer {      // this entity needs an ID:     @Id     @Column(name="useranswer_id")     @GeneratedValue(strategy = GenerationType.AUTO)     private Long id;      @ManyToOne     @JoinColumn(name = "user_id")     private User user;      @ManyToOne     @JoinColumn(name = "question_id")     private Question question;      @Column(name = "response")     private String response;      //getter and setter  } 

The Hibernate documentation explains that. Read it. And also read the javadoc of the annotations.

Answer 2

There are many answers and all are correct as well. But unfortunately none of them have a clear explanation.

The following works for a non-primary key mapping as well.

Let's say we have parent table A with column 1 and another table, B, with column 2 which references column 1:

@ManyToOne @JoinColumn(name = "TableBColumn", referencedColumnName = "TableAColumn") private TableA session_UserName; 

@ManyToOne @JoinColumn(name = "bok_aut_id", referencedColumnName = "aut_id") private Author bok_aut_id;