How to do $lookup in an array's field and add the foreign table's content in the same query?

I am trying to make a query in which I have a db_task query that contains the task id and the users assigned to it. I have to fetch the user details which are present in db_user collection and add the incoming details into the same document.

db_task

{
    "_id" : ObjectId("5d8b522d0cf2579c57bc8ce0"),
    "users" : [ 
        {
            "user_id" : ObjectId("5d8b522d0cf2579e27bc8ce3"),
        "is_finished" : false
        }, 
        {
            "user_id" : ObjectId("5d6f6d25e079b9fb7d858236"),
            "is_finished" : false
        }
    ]
}

The users field the users who are assigned that task, I want to do a lookup on the db_user query which would get me the details inside the same embedded document.

db_user

{
    "_id" : ObjectId("5d8b522d0cf2579e27bc8ce3"),
    "first_name" : "Harry",
    "last_name" : "Paul"
},

{
    "_id" : ObjectId("5d6f6d25e079b9fb7d858236"),
    "first_name" : "Aaron",
    "last_name" : "Potter"
}

I tried to do $lookup on the db_user table with "users.user_id" but that would fetch me a new field and then I tried to concatenate those arrays with "$concatArrays" but the result still wasn't what I expected.

I want to get the output in a format something like this

db_task

{
    "_id" : ObjectId("5d8b522d0cf2579c57bc8ce0"),
    "users" : [ 
        {
            "user_id" : ObjectId("5d8b522d0cf2579e27bc8ce3"),
            "is_finished" : false,
            "user_info":{
                 "first_name" : "Harry",
                 "last_name" : "Paul"
              }
        }, 
        {
            "user_id" : ObjectId("5d6f6d25e079b9fb7d858236"),
            "is_finished" : false,
            "user_info":{
                  "first_name" : "Aaron",
                   "last_name" : "Potter"
              }
        }
    ]
}

Altough they're working, the provided solutions, with unwind and group, can be expensive in resources. Here's a better solution in only two stages :

db.db_task.aggregate([
  {
    $lookup: {
      from: "db_user",
      localField: "users.user_id",
      foreignField: "_id",
      as: "usersInfos"
    }
  },
  {
    $project: {
      users: {
        $map: {
          input: "$usersInfos",
          as: "ui",
          in: {
            $mergeObjects: [
              "$$ui",
              {
                $arrayElemAt: [
                  {
                    $filter: {
                      input: "$users",
                      as: "users",
                      cond: {
                        $eq: [
                          "$$users.user_id",
                          "$$ui._id"
                        ]
                      }
                    }
                  },
                  0
                ]
              }
            ]
          }
        }
      }
    }
  }
])

Will output

[
  {
    "_id": ObjectId("5d8b522d0cf2579c57bc8ce0"),
    "users": [
      {
        "_id": ObjectId("5d6f6d25e079b9fb7d858236"),
        "first_name": "Aaron",
        "is_finished": false,
        "last_name": "Potter",
        "user_id": ObjectId("5d6f6d25e079b9fb7d858236")
      },
      {
        "_id": ObjectId("5d8b522d0cf2579e27bc8ce3"),
        "first_name": "Harry",
        "is_finished": true,
        "last_name": "Paul",
        "user_id": ObjectId("5d8b522d0cf2579e27bc8ce3")
      }
    ]
  }
]

Note : as proposed by @Valijon, you can add a $project stage if you need to slighty re-arrange from here.