db={"Books":[{"BOOK_ID":"100","BOOK_NAME":"Book 1","BOOK_DESC":"abcd",},{"BOOK_ID":"101","BOOK_NAME":"Book 2","BOOK_DESC":"efgh",},{"BOOK_ID":"102","BOOK_NAME":"Book 3","BOOK_DESC":"ijkl",}],"Scores":[{"BOOK_ID":"100","BOOK_CATEGORY":"kids","BOOK_SCORE":"6",},{"BOOK_ID":"100","BOOK_CATEGORY":"Educational","BOOK_SCORE":"8",},{"BOOK_ID":"101","BOOK_CATEGORY":"Kids","BOOK_SCORE":"6",},{"BOOK_ID":"101","BOOK_CATEGORY":"Fantasy","BOOK_SCORE":"7",}]}

db.Books.aggregate([{"$lookup":{"from":"Scores","pipeline":[{"$match":{"BOOK_CATEGORY":"kids","BOOK_SCORE":"6"}}],"localField":"BOOK_ID","foreignField":"BOOK_ID","as":"SCORES"}},{$addFields:{SCORES:"$$REMOVE","BOOK_SCORE":{$cond:[{"$ifNull":[{"$arrayElemAt":["$SCORES",0]},false]},{$getField:{field:"BOOK_SCORE",input:{"$arrayElemAt":["$SCORES",0]}}},"$$REMOVE"]},"BOOK_CATEGORY":{$cond:[{"$ifNull":[{"$arrayElemAt":["$SCORES",0]},false]},{$getField:{field:"BOOK_CATEGORY",input:{"$arrayElemAt":["$SCORES",0]}}},"$$REMOVE"]},}}])