code: function(_params) {
        return this.$action(function() {
          var request = { method: 'GET', url: this.$url('fetchMany'), params: this.$params };
  
          if(_params) {
            request.params = request.params ? extend(request.params, _params) : _params;
          }
  
          // TODO: check that collection is bound.
  
          this
            .$dispatch('before-fetch-many', [request])
            .$send(request, function(_response) {
              this
                .$unwrap(_response.data)
                .$dispatch('after-fetch-many', [_response]);
            }, function(_response) {
              this.$dispatch('after-fetch-many-error', [_response]);
            });
        });
      }
docs: @memberof CollectionApi#
  
  @description Begin a server request to populate collection. This method does not
  clear the collection contents by default, use `$refresh` to reset and fetch.
  
  This method is for use in collections only.
  
  @param {object|function} _params Additional request parameters, not stored in collection,
  if a function is given, then it will be called with the request object to allow requet customization.
  @return {CollectionApi} self