Return a single object from Aggregate Method Mongoose

I have a dynamic model 'Dialog' model in which a field called lecture is sometimes available sometimes not, and the Lecture model is defined on different nodejs server. I want to populate the result of the lecture in the dialog as well. so I try using populate but it gives a Schema not register error. So I am trying to using $lookup to join data from different collections. But, the Aggregate method returns an array and I need an object. I have tried accessing using arr[0] using index zero. But I have a method defined on the Dialog model. when I try accessing it throws an error.

  const resp = await DialogModel.aggregate([
        {
            $match: {
                $and: query
            }
        },
        {
            $lookup: {
                from: "lectures",
                localField: "lecture",
                foreignField: "_id",
                as: "lectureDetail"
            }
        },
        {
            $unwind: {
                path: "$lectureDetail",
                preserveNullAndEmptyArrays: true
            }
        }
    ]);

Here is current out:

[
  {
    _id: 60ab86e1671c602015741955,
    lecture: 607d4d3de9b8be1f29e56e94,
    lectureDetail: {
      _id: 607d4d3de9b8be1f29e56e94,
      course: 6076ba3a01060327b3231a3c,
      owner: 607434cd63b75949f61cc065,
      name: 'Lecture 11',
      startDate: 2021-05-24T07:00:00.000Z,
      endDate: 2021-05-24T08:00:00.000Z
    }
  }
]

Here is the expected output:

{
  _id: 60ab86e1671c602015741955,
  lecture: 607d4d3de9b8be1f29e56e94,
  lectureDetail: {
    _id: 607d4d3de9b8be1f29e56e94,
    course: 6076ba3a01060327b3231a3c,
    owner: 607434cd63b75949f61cc065,
    name: 'Lecture 11',
    startDate: 2021-05-24T07:00:00.000Z,
    endDate: 2021-05-24T08:00:00.000Z
  }
}

If I try getting access using the index, and call method on it. method Defined on my Model through an error of is not a function.

Here is my query. Kindly guide me on how to solve this issue.