How to unit test grails service method which uses mongo query?

I have a service with a method called getEmployeeSalary that i want to unit test but i can not mock and query the employee collections

private List getEmployeeSalary(String department, Date from){

     def db = grailsApplication.mainContext.mongo.getDB(grailsApplication.config.grails.mongo.databaseName)

     List employeeSalaryByDepartment = db.employee.aggregate([
                [ '$match' : [ department: department, 
                  dateCreated: ['$gte': from] ] ],
                [
                   $group' : [
                            '_id': "\$department",
                             'sum': [ '$sum': "\$salary" ]
                            ]
                ]
        ]).results()*.values()

   return employeeSalaryByDepartment
}