Query Posts by user -> paginated results and a total count.

mongodb-aggregation.js

// Aggregation using MongoDB
/**
 * Query posts by user -> paginated results and a total count.
 * @param userId {ObjectId} ID of user to retrieve posts for
 * @param skip {Number} First row to return in results
 * @param limit {Number} Last row to return in results
 * @param sort {Object} sort query object
 * @returns {Object} Object -> `{ rows, count }`
 */
function queryPostsByUser (userId, skip, limit, sort) {
  
  // aggregation query
  const query = [ 
    // more lookups go here if you need them
    // we have a many-to-one from Post -> user
    { $lookup: {
      from: 'users',
      localField: 'user',
      foreignField: '_id',
      as: 'user'
    } },
    // each post has a single user (author) so flatten it using $unwind
    { $unwind: '$user' },
    // filter the results by our userId
    { $match: { 'user._id': userId } }
  ]

  query.push(
    { $facet: {
      metadata: [
        { $group: { 
          _id: null,
          total: { $sum: 1 }
        }},
      ],
      data: [ 
        { $sort: sort },
        { $skip: skip },
        { $limit: limit },
      ]
    }
    { $project: { 
        data: 1,
        // Get total from the first element of the metadata array 
        total: { $arrayElemAt: [ '$metadata.total', 0 ] }
      }
    }
  )

  //  { total: 10000,  data: [ { x }, { y }, ... ]  } // output
  return Post
    .aggregate(query)
    .then(([{ total, data }]) => ({ count, rows }))
}