withPivot(columns) {
      if (!Array.isArray(columns)) columns = [columns];
      this.pivotColumns = this.pivotColumns || [];
      push.apply(this.pivotColumns, columns);
    }
Sets the pivot column names to be retrieved along with the current model. This allows for
additional fields to be pulled from the joining table.

@param {string|string[]} columns Extra column names to fetch.
@return {undefined}