Use attr_compare <attribute list>
to declare the attributes which should be compared, in their order of precedence.
Attributes may be nil. nil attributes sort earlier than non-nil to match the SQL behavior for NULL.
Consider this value class that holds full names:
class FullName
include Comparable
attr_reader :first, :middle, :last, :suffix
def initialize(first, middle, last, suffix = nil)
@first = first
@middle = middle
@last = last
@suffix = suffix
end
def <=>(other)
(last <=> other.last).nonzero? ||
(first <=> other.first).nonzero? ||
(middle <=> other.middle).nonzero? ||
suffix <=> other.suffix
end
def to_s
no_suffix = [first.presence, middle.presence, last.presence].compact.join(' ')
[no_suffix, suffix.presence].compact.join(', ')
end
endYou can see that the <=> method isn't very DRY, and as shown it doesn't even work with nil.
(That's just too ugly to show.)
Here it is using the gem. Only the 2 lines with the comments are needed.
require 'attr_comparable'
require 'active_support'
class FullName
include AttrComparable # AttrComparable automatically includes Comparable
attr_compare :last, :first, :middle, :suffix # will be compared in this precedence order
attr_reader :first, :middle, :last, :suffix
def initialize(first, middle, last, suffix = nil)
@first = first
@middle = middle
@last = last
@suffix = suffix
end
def to_s
no_suffix = [first.presence, middle.presence, last.presence].compact.join(' ')
[no_suffix, suffix.presence].compact.join(', ')
end
end>> mom = FullName.new("Kathy", nil, "Doe")
>> dad = FullName.new("John", "Q.", "Public")
>> junior = FullName.new("John", "Q.", "Public", "Jr.")
>> junior > dad
=> true
>> [junior, mom, dad].sort.map &:to_s
=> ["Kathy Doe", "John Q. Public", "John Q. Public, Jr."]