db={"users":[{"user_id":101,"name":"Alice Smith","email":"alice.smith@example.com"},{"user_id":102,"name":"Bob Johnson","email":"bob.johnson@example.com"},{"user_id":103,"name":"Charlie Brown","email":"charlie.brown@example.com"},{"user_id":104,"name":"Diana Prince","email":"diana.prince@example.com"}],"sales":[{"_id":1,"user_id":101,"products":[{"name":"laptop","price":1200},{"name":"mouse","price":25}],"total":1225},{"_id":2,"user_id":102,"products":[{"name":"headphones","price":100},{"name":"microphone","price":150}],"total":250},{"_id":3,"user_id":103,"products":[{"name":"monitor","price":300},{"name":"keyboard","price":75}],"total":375},{"_id":4,"user_id":104,"products":[{"name":"tablet","price":500},{"name":"stylus","price":50}],"total":550}]}

db.sales.aggregate([{"$lookup":{"from":"users","let":{"userId":"$user_id"},"pipeline":[{"$match":{"$expr":{"$and":[{"$eq":["$user_id","$$userId"]}]}}},{"$project":{"_id":0,"email":1}},{"$limit":1}],"as":"users"}}])