docs: function (id) {
              var pk = this.primaryKey;
              pk = pk.length === 1 ? pk[0] : pk;
              var q = {};
              if (isArray(id) && isArray(pk)) {
                  if (id.length === pk.length) {
                      pk.forEach(function (k, i) {
                          q[k] = id[i];
                      });
                  } else {
                      throw new ModelError("findById : ids length does not equal the primaryKeys length.");
                  }
              } else {
                  q[pk] = id;
              }
              return this.filter(q).one();
          }