Discoveries in Ruby(and Rails): Give classes the ability to be compared using the Comparable module

The equality-test operators is a set of members that consist of ==(equals), !=(not equal), > (greater than), <(less than), >=(greater or equal), and <= (less or equal), with the most common being the == operator.

We can see that with both Integer and Float which are subclasses of the Numeric class. Objects of both classes can be compared using the equality-test operators because they include the Comparable module, and override the comparison method <=>, also called the spaceship operator or spaceship method.

Let's imagine in an e-commerce sight we have Products. A product can have many different Price per customer/conuntry. The modeling might look like this

class Price
  attr_accessor :amount_cents

  def initialize(amount_cents)
    self.amount_cents = amount_cents
  end
end

Let's imagine that we need to compare the price of two products(for whatever reason we need to). To compare the amount_cents we might do something like

if price_1.amount_cents < price_2.amount_cents
  puts "price_1 is less than price_2"
elsif price_1.amount_cents > price_2.amount_cents
  puts "price_1 is greater than price_2"
else
  puts "both prices are equal"
end

To compare the price objects we simply need to do so on the amount_cents attribute, since it's an instance of Numeric(either float or integer) it can be compared using the equality-test operators mentioned above.

But, we cannot do something like this

if price_1 < price_2
  puts "price_1 is less than price_2"
elsif price_1 > price_2
  puts "price_1 is greater than price_2"
else
  puts "both prices are equal"
end

Because our class Price does not include the Comparable module, thus it does not support this. If we try to we get

 undefined method `<' for #<Price:0x00007fbd98992de0 @amount_cents=10> (NoMethodError)              

The ruby interpreter tells us that the class does not have support for the < method. Because we did not define this.

Lo! and behold the Comparable module in action.

Let's allow our Price class to work nice with the equality-test operators. To do that we need to do

• include the Comparable module
• override the <=> method

Let's rewrite the Product class to support that.

class Price
  include Comparable
  attr_accessor :amount_cents

  def initialize(amount_cents)
    self.amount_cents = amount_cents
  end

  def <=>(other_price)
    if self.amount_cents < other_price.amount_cents
      -1
    elsif self.amount_cents > other_price.amount_cents
      1
    else
      0
    end
  end
end

Notice the return value for the <=> method.

• -1 means that it's less than the passed argument.
• 1 means that it's greater than the passed argument.
• 0 means both prices are equal.

Now, we can run this code

if price_1 < price_2
  puts "price_1 is less than price_2"
elsif price_1 > price_2
  puts "price_1 is greater than price_2"
else
  puts "both prices are equal"
end

and get the output

price_1 is less than price_2
=> nil

Including the Comparable module provides us with more methods more than just the equality-test operators. For example, using the Price class determine if a price is between two other price

price_1 = Price.new 100
price_2 = Price.new 200
price_3 = Price.new 150

price_3.between? price_1, price_2

The output would be true

We can also sort them!

[price_3, price_1, price_2].sort
[<Price:0x00007fa65e225d38 @amount_cents=100>,                          
 <Price:0x00007fa65e0be9b8 @amount_cents=150>,                          
 <Price:0x00007fa661b965e0 @amount_cents=200>]      

Note

We can piggy-back the method call to the attribute itself. Since it's an instance of Numeric. We can shorten the method to

class Price
  include Comparable
  attr_accessor :amount_cents

  def initialize(amount_cents)
    self.amount_cents = amount_cents
  end

  def <=>(other_price)
    self.amount_cents <=> other_price.amount_cents
  end
end

Conclusion

We can allow our classes to work well with the equality-test family members and save ourselves much time trying to access attributes for comparison. And as a side-effect our Ruby code becomes more natural and ruby-ish, and we type less :).

Thanks for reading, and Happy Coding!.

Resources

• Comparable docs