OneToMany

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

Annotation Attributes

OneToMany 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.

OneToMany Example

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

public class Department {

    private String mName;

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

    // ...

}

public class Manager extends Employee {

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

    // ...

}

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