[{_id:"1",query:"How to solve...?",answer:[],is_solved:false},{_id:"2",query:"How to solve...?",answer:[],is_solved:true},{_id:"3",query:"How to solve...?",answer:[],is_solved:true},{_id:"4",query:"How to solve...?",answer:[],is_solved:true},]

db.collection.aggregate([{"$facet":{"not_solved":[{"$match":{"is_solved":false}}],"solved":[{"$match":{"is_solved":true}},{"$limit":1}]}},{"$project":{"result":{"$concatArrays":["$not_solved","$solved"]}}}])