jpa-join-column

Posted on: 2019-10-08 2019-11-19
Tags: Hibernate, JPA

@JoinColumn Annotation Explained

1. Introduction

The annotation javax.persistence.JoinColumn marks a column for as a join column for an entity association or an element collection.

In this quick tutorial, we’ll show some examples of basic @JoinCloumn usage.

**2. @OneToOne Mapping Example**

The @JoinColumn annotation combined with a @OneToOne mapping indicates that a given column in the owner entity refers to a primary key in the reference entity:

@Entity
public class Office {
    @OneToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "addressId")
    private Address address;
}

The above code example will create a foreign key linking the Office entity with the primary key from the Address entity. The name of the foreign key column in the Office entity is specified by name property.

**3. @OneToMany Mapping Example**

When using a @OneToMany mapping we can use the mappedBy parameter to indicate that the given column is owned by another entity.

@Entity
public class Employee {

    @Id
    private Long id;

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "employee")
    private List<Email> emails;
}

@Entity
public class Email {

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "employee_id")
    private Employee employee;
}

In the above example, Email (the owner entity) has a join column employee_id that stores the id value and has a foreign key to the Employee entity.

4. @JoinColumns

In situations when we want to create multiple join columns we can use the @JoinColumns annotation:

@Entity
public class Office {
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumns({
        @JoinColumn(name="ADDR_ID", referencedColumnName="ID"),
        @JoinColumn(name="ADDR_ZIP", referencedColumnName="ZIP")
    })
    private Address address;
}

The above example will create two foreign keys pointing to ID and ZIP columns in the Address entity:

5. Conclusion

In this article, we’ve learned how to use the @JoinColumn annotation. We’ve shown examples how to create both single entity association and element collection.

As always, all source code is available over on GitHub.

getdocs

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