def where_values(attribute, type = :integer)
      return @where_values[attribute] if @where_values.key?(attribute)
      @where_values[attribute] ||= array_of_values(@wheres[attribute], type)
    end
def where_values(attribute, type = :integer)
      return @where_values[attribute] if @where_values.key?(attribute)
      @where_values[attribute] ||= array_of_values(@wheres[attribute], type)
    end
Retrieve the values for the given attribute from the where statements
 Returns nil if no the attribute is not present
 @param attribute: the attribute name
 @param type: the type to be retrieved, :integer or :string (defaults to :integer)