def where_value(attribute, type = :integer)
      values = where_values(attribute, type)
      return nil unless values && values.size == 1
      values.first
    end
def where_value(attribute, type = :integer)
      values = where_values(attribute, type)
      return nil unless values && values.size == 1
      values.first
    end
Retrieve the single value for the given attribute from the where statements
 Returns nil if the attribute is not present, or if it contains multiple values
 @param attribute: the attribute name
 @param type: the type to be retrieved, :integer or :string (defaults to :integer)