db={"employee_details":[{"_id":"100","department_id":"1","employee_fname":"Emp1"},{"_id":"101","department_id":"2","employee_fname":"Emp2"},{"_id":"102","department_id":"3","employee_fname":"Emp3"},{"_id":"103","department_id":"1","employee_fname":"Emp4"},{"_id":"104","department_id":"2","employee_fname":"Emp5"},{"_id":"105","department_id":"3","employee_fname":"Emp6"},{"_id":"106","department_id":"1","employee_fname":"Emp7"},{"_id":"107","department_id":"2","employee_fname":"Emp8"},{"_id":"108","department_id":"3","employee_fname":"Emp9"},{"_id":"109","department_id":"1","employee_fname":"Emp10"},{"_id":"110","department_id":"2","employee_fname":"Emp11"},{"_id":"111","department_id":"3","employee_fname":"Emp12"},{"_id":"112","department_id":"1","employee_fname":"Emp13"},{"_id":"113","department_id":"2","employee_fname":"Emp14"},],"departments":[{"_id":"1","dept_name":"Dept1",},{"_id":"3","dept_name":"Dept3"},{"_id":"2","dept_name":"Dept2",},]}

db.departments.aggregate([{$sort:{"dept_name":1}},{$lookup:{from:"employee_details",localField:"_id",foreignField:"department_id",as:"Employee"}},{$unwind:"$Employee"},{$addFields:{tmp:{$mergeObjects:[{Department:"$$ROOT"},"$Employee"]}}},{$project:{"tmp.Department.Employee":0}},{$addFields:{"tmp.Department":["$tmp.Department"]}},{$replaceRoot:{newRoot:"$tmp"}},{$limit:30}])