MongoDb EMongoDocument YII，集合关系

I have 2 tables in MongoDb, Both are related to each other.

For example Employee and Department. Employee is related to Department.

Employee
{
    '_id' : ID_OF_EMPLOYEEE,
    'Name' : NAME_OF_EMPLOYEEE
    'dept_id' : ID_OF_EMPLOYEES_DEPT
}

DEPT
{
    '_id' : ID_OF_EMPLOYEEE,
    'Name' : NAME_OF_DEPT
}

Now, I want to create a YII Relation which will give me name of Employee's Department(Like what we do in Relation DB). it should look like

 $dept = $employee->department

Please give me a solution

Thanks.