db={rooms:[{_id:ObjectId("62db88affeb2d64c1b818d8b"),seats:54,amenities:["AC","Water"],price:5000},{_id:ObjectId("62db8927feb2d64c1b818d8c"),seats:52,amenities:["Water"],price:52000},{_id:ObjectId("62db893afeb2d64c1b818d8d"),seats:520,amenities:["AC","Water"],price:52000},{_id:ObjectId("62db894efeb2d64c1b818d8e"),seats:529,amenities:["AC","Water"],price:9000}],customers:[{_id:ObjectId("62db8c69feb2d64c1b818d91"),customerName:"John",date:"20-04-2020",startTime:"7PM",endTime:"10PM",roomID:ObjectId("62db893afeb2d64c1b818d8d")},{_id:ObjectId("62db8c92feb2d64c1b818d92"),customerName:"Harry",date:"18-04-2020",startTime:"7PM",endTime:"10PM",roomID:ObjectId("62db8927feb2d64c1b818d8c")}]}

db.rooms.aggregate([{"$lookup":{"from":"customers","localField":"_id","foreignField":"roomID","as":"customer"}},{$addFields:{reservation:{$cond:[{$eq:["$customer",[]]},"Not booked","Booked"]}}}])