def <<(key)
      column = Column.new(@table, self, key)
      return if is_blank?(column.key) || key?(key)

      @column_indexes[column.key] = @columns.size
      @column_indexes[@columns.size] = column
      @columns_by_key[column.key] = column
      @columns << column
    end
def <<(key)
      column = Column.new(@table, self, key)
      return if is_blank?(column.key) || key?(key)

      @column_indexes[column.key] = @columns.size
      @column_indexes[@columns.size] = column
      @columns_by_key[column.key] = column
      @columns << column
    end
Add a new Column with +key+