groupBy(column) {

    let columns;
    let transformation;

    if (typeof column === 'function') {
      columns = utilities.getFunctionParameters(column);
      transformation = column;
    } else {
      columns = [column];
      transformation = v => `${v}`;
    }

    this._command = {
      type: 'groupBy',
      data: {
        columnNames: columns,
        transformation: transformation
      }
    };

    return new Composer(this.Model, this).aggregate(column);

  }
Groups by a specific field, or a transformation on a field
@param {String} column The column to group by