code: function count(coll, query, options, callback) {
    if (typeof options === 'function') (callback = options), (options = {});
    options = Object.assign({}, options);
    options.collectionName = coll.s.name;
  
    options.readPreference = resolveReadPreference(options, {
      db: coll.s.db,
      collection: coll
    });
  
    let cmd;
    try {
      cmd = buildCountCommand(coll, query, options);
    } catch (err) {
      return callback(err);
    }
  
    executeCommand(coll.s.db, cmd, options, (err, result) => {
      if (err) return handleCallback(callback, err);
      handleCallback(callback, null, result.n);
    });
  }
docs: Count the number of documents in the collection that match the query.
  
  @method
  @param {Collection} a Collection instance.
  @param {object} query The query for the count.
  @param {object} [options] Optional settings. See Collection.prototype.count for a list of options.
  @param {Collection~countCallback} [callback] The command result callback