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Ahmad khattab
Ahmad khattab

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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
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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
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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
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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)              
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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
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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
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and get the output

price_1 is less than price_2
=> nil
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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
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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>]      
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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
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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!.

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Top comments (5)

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rockwell profile image
Ahmad khattab

Damn. Yea you're right. We can piggy-back function to Ruby. Since it's a Numeric.

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rockwell profile image
Ahmad khattab

Just added it to the post!!. Thanks for pointing that out!

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rockwell profile image
Ahmad khattab

Funny thing is, I am. Just couldn't think of a better example to illustrate the thingi!.