db={"person":[{_id:"5ff93b43535bera64de4",first_name:"some name",last_name:"some name",dept:"department1",}],"department":[{_id:1,dept_name:"department1",dept_descp:"this is description",books:[{book_name:"book1",subject:"subject1"},{book_name:"book2",subject:"subject2"}]}]}

db.person.aggregate([{$match:{"_id":"5ff93b43535bera64de4"}},{$lookup:{from:"department",localField:"dept",foreignField:"dept_name",as:"ndept"}},{$addFields:{"books":{$arrayElemAt:["$ndept.books",0]}}}])