db={"user_activity":[{"_id":ObjectId("5f4ddd8c64d8905be4d2b9a2"),"userId":"1e40v0b1j5","date":ISODate("2020-09-01T05:35:08.822Z"),"itemId":"1eh42irfo45m","action":"vote","vote":"neutral"},{"_id":ObjectId("5f4ddd8c64d8905be4d2b9a4"),"userId":"1e40v0b1j5","date":ISODate("2020-09-01T05:35:08.842Z"),"itemId":"1eh42irg7kar","action":"vote","vote":"hate"},{"_id":ObjectId("5f4ddd8c64d8905be4d2b9a6"),"userId":"1e40v0b1j5","date":ISODate("2020-09-01T05:35:08.861Z"),"itemId":"1eh42irfo45m","action":"comment","commentId":"1eh42irhnzvh"},{"_id":ObjectId("5f4ddd8c64d8905be4d2b9a7"),"userId":"1e4176ttr8","date":ISODate("2020-09-01T05:35:08.872Z"),"itemId":"1eh42irg7kar","action":"comment","commentId":"1eh42iri6yie"},{"_id":ObjectId("5f4ddd8c64d8905be4d2b9a8"),"userId":"1e40v0b1j5","date":ISODate("2020-09-01T05:35:08.883Z"),"itemId":"1eh42irg7kar","action":"vote","commentId":"1eh42iri6yie","vote":1}],"items":[{"_id":"1eh42irfo45m","type":"poll","votes_count":1,"info":{"author":{"nickname":"literakl","id":"1e40v0b1j5"},"caption":"First question","slug":"first-question","published":false,"date":ISODate("2020-09-01T05:35:08.792Z"),"picture":"picture.png","tags":null},"data":{"votes":{"neutral":1,"trivial":0,"dislike":0,"hate":0}},"comments":{"count":1,"last":ISODate("2020-09-01T05:35:08.853Z")}},{"_id":"1eh42irg7kar","type":"poll","votes_count":1,"info":{"author":{"nickname":"Jana","id":"1e417bgivc"},"caption":"Second question","slug":"second-question","published":false,"date":ISODate("2020-09-01T05:35:08.807Z"),"picture":"picture.png","tags":null},"data":{"votes":{"neutral":0,"trivial":0,"dislike":0,"hate":1}},"comments":{"count":1,"last":ISODate("2020-09-01T05:35:08.869Z")}}]}

db.user_activity.aggregate([{$match:{"userId":"1e40v0b1j5"}},{$lookup:{from:"items",localField:"itemId",foreignField:"_id",as:"item_doc"}},{$replaceRoot:{newRoot:{$mergeObjects:[{$arrayElemAt:["$item_doc",0]},"$$ROOT"]}}},{$project:{userId:1,date:1,itemId:1,action:1,vote:1,"info.slug":1,"info.caption":1,comments:1}}])