ManyToMany

The ManyToMany annotation is used to define a many-to-many relationship between two domain entities. The annotation is placed in each class which make up the relationship on the field which models it. For example, if class Foo has a many-to-many relationship with class Bar, Foo will contain a collection of Bar and vice versa. The annotation would then be placed on each of these fields in the respective classes.

Annotation Attributes

ManyToMany has five required attributes that are used to define the relationship and how it should be stored.

tableName: indicates the name of the many-to-many table which this relationship will be stored in.

keyField: indicates the name of the field identifying this class's side (relative to the annotation) of the relationship, typically its primary key.

foreignField: indicates the name of the field identifying the associated class's side (relative to the annotation) of the relationship, typically the primary key of the associated class.

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

ManyToMany` Example

The code below shows how a many-to-many relationship is established between the two classes Employee and Department using the ManyToMany annotation. Let's assume that an Employee can work in multiple Departments and a Department has multiple Employees. Let's also assume that an Employee is uniquely identified by his or her employee ID and a Department is uniquely identified by its name.

public class Employee {

    private long mEmpId;

    @ManyToMany(name="worksIn", keyField="mEmpId", foreignField="mName", tableName="emp_dept")
    private List<Department> mDepartments;

    // ...

}

public class Department {
    
    @PrimaryKey(autoincrement=false)
    private String mName;

    @ManyToMany(name="employs", keyField="mName", foreignField="mEmpId", tableName="emp_dept")
    private List<Employee> mEmployees;

    // ...

}

A many-to-many relationship has now been identified between Employee and Department such that the relationship data will be stored in the table emp_dept.