[{_id:20,housename:"white",adress:"St1",people:[{name:"Jon",age:23},{name:"Ann",age:50},{name:"Pat",age:20},{name:"Helen",age:15}]},{_id:30,housename:"black",adress:"St2",people:[{name:"Sam",age:23},{name:"Craig",age:50},{name:"Paul",age:20},{name:"John",age:15}]}]

db.collection.aggregate([{$match:{"_id":20}},{"$addFields":{people:{$filter:{input:"$people",as:"p",cond:{$gt:["$$p.age",21]}}}}}])