ManyToOne

The ManyToOne annotation is used to define a many-to-one relationship between two domain entities. The annotation is placed in the "many" side of the relationship on the field which models it. Conversely, a OneToMany annotation is placed in the "one" side of the relationship. For example, if class Foo has a many-to-one relationship with class Bar, Foo will contain a Bar field and Bar will contain a collection of Foo. The ManyToOne annotation would then be placed on the field in Foo and the OneToMany annotation on the field in Bar.

Annotation Attributes

ManyToOne has three required attributes that are used to define the relationship and how it should be represented.

column: indicates the name of the column representing the foreign key in this relationship.

name: indicates the name of the relationship. Names are used to uniquely identify relationships within a class and provide descriptive metadata.

ManyToOne Example

The code below shows how a many-to-one relationship is established between the two classes Manager and Department using the ManyToOne and OneToMany annotations. Let's assume that a Manager can manage one Department and a Department can be managed by multiple Managers.

public class Manager extends Employee {

    @ManyToOne(name="manages", column="dept")
    private Department mDepartment;

    // ...

}

public class Department {

    private String mName;

    @OneToMany(name="managedBy", column="dept")
    private List<Manager> mManagers;

    // ...

}

The example above establishes a many-to-one relationship between Manager and Department. The column dept in the Manager table acts as a foreign key referencing the Department a Manager is responsible for.