db={users:[{"_id":"5fcdcc14da26c80a6de06754","first_name":"James","last_name":"Hales","username":"jim","timestamp":1607322644202},{"_id":"5fcdd0075dc42a0d5c7ec813","first_name":"Sames","last_name":"Karun","username":"Sames","timestamp":1607323655644}],site:[{"_id":ObjectId("6011021851781d3d9841e09d"),"user_id":ObjectId("5fcdcc14da26c80a6de06754"),"picture":[{"_id":ObjectId("6011021851781d3d9841e09e"),"url":"http://localhost:3000/files/ou93r0cfjde0cat.jpg","width":250,"height":500,"preview":"skjdfbsdjvns;svnsdvisdvnvsdjvnsdivsdkvsdivsdjvnsdjvsdvsdvgjsvnsvhi"}],"status":"Active","timestamp":1611727384083,"expire_at":ISODate("2021-01-27T06:03:04.083Z")},{"_id":ObjectId("6011021851781d3d9841e09e"),"user_id":ObjectId("5fcdcc14da26c80a6de06754"),"picture":[{"_id":ObjectId("6011021851781d3d9841e09e"),"url":"http://localhost:3000/files/ou93r0cfjee0bat.jpg","width":250,"height":500,"preview":"skjdfbsdjvns;svnsdvisdvnvsdjvnsdivsdkvsdivsdjvnsdjvsdvsdvgjsvnsvhi"}],"status":"Active","timestamp":1611727384083,"expire_at":ISODate("2021-01-27T06:03:04.083Z")}]}

db.site.aggregate([{"$lookup":{"from":"users","localField":"site","foreignField":"user_id","as":"users"}}])