MongoDb mongoose aggregatePagination returns multiple same documents over different pages

Question

I am facing an issue with mongodb and mongoose with pagination. I am trying to query over a set of Tutors and get those who are matching the query in a paginated and sorted (by updatedDate) result. But the fact is I get multiple same documents over different pages ... I would like it to return a set of unique documents. Here is part of my function (the rest is just to build the query from the request body):

exports.search = (req, res) => {

    var options = { page: page, limit: perPage, sortBy: { updatedDate: -1 }}

    const aggregate = Tutor.aggregate([
  {
    "$geoNear":
      {
        "near":
          {
            "type": "Point",
            "coordinates": [lon1, lat1]
          },
        "distanceField": "distance",
        "spherical": true,
        "maxDistance": radius
      }
  },
  {
    $match: match
  }
]);

Tutor
.aggregatePaginate(aggregate, options, function (err, result, pageCount, count) {
  if (err) {
    return res.status(400).send(err);
  }
  else {
    var opts = [
      { path: 'levels', select: 'name' },
      { path: 'subjects', select: 'name' },
      { path: 'assos', select: 'name' }
    ];
    Tutor
      .populate(result, opts)
      .then(result2 => {
        return res.send({
          page: page,
          perPage: perPage,
          pageCount: pageCount,
          documentCount: count,
          tutors: result2
        });
      })
      .catch(err => {
        return res.status(400).send(err);
      });
  }
})
};

Now, imagine I am querying page 1 with a limit per page of 8. I 've got back my 8 documents correctly, all different. But when I am querying page 2, with the same limit per page, half of the documents were already returned in page 1 ! Would you know why is that so ? Thank you !

EDIT

I figured it was the sortBy that makes it happen. If i remove it, everything works correctly. I need it though ...

Answer 1

After doing alot of research. I found this solution. Hope it will solve your issue.

Jobs.aggregatePaginate(aggregateQuery, { page: 1, limit: 10, sort: { 'createdAt': 'desc' } }, function (){});