docs: function (source) {
              if (!Array.isArray(source)) {
                  source = [source];
              }
              if (!source || !source.length) {
                  throw new QueryError("No source specified for the query");
              }
              return " " + source.map(
                      function (s) {
                          return this.__tableRef(s);
                      }, this).join(this._static.COMMA_SEPARATOR);
          }