db={"post":[{"_id":"61f7a933b209737de4cc2657","author":"61d30188cf93e83e08d14112","title":"Some title","createts":1643620659355}],"user":[{"_id":"61d30188cf93e83e08d14112","nickname":"Bob","link":"bobspage"}]}

db.post.aggregate([{"$lookup":{"from":"user","localField":"author","foreignField":"_id","as":"author"}},{"$addFields":{"author":{"$arrayElemAt":["$author",0]}}}])