db={"problems":[{"problemId":1,"title":"dummy 1",},{"problemId":2,"title":"dummy 2",},{"problemId":3,"title":"dummy 3",}],"submissions":[{"submissionId":1,"status":"AC","problemId":1,},{"submissionId":2,"status":"AC","problemId":1,},{"submissionId":3,"status":"WA","problemId":2,},{"submissionId":4,"status":"WA","problemId":1,},{"_id":5,"status":"AC","problemId":2,},{"_id":6,"status":"WA","problemId":2,}]}

db.problems.aggregate([{"$lookup":{"from":"submissions","localField":"problemId","foreignField":"problemId","as":"submission_docs"}}])