[{u1:"xx",u2:"gg"},{u1:"yy",u2:"Nk"},{u1:"zz",u2:"hh"},{u1:"ya",u2:"hj"},{u1:"ab",u2:"jd"}]

db.collection.aggregate([{"$project":{_id:0,user:["$u1","$u2"]}},{"$unwind":"$user"}])