How do I extract and sort the instance variables of one class, that are stored in an array inside of an instance variable in another class using ruby? - arrays

Question

While learning more about Ruby, I'm currently running into a wall. I'm trying to extract and sort the value of an instance variable of one class, that's stored in an array inside an instance variable in another class. I seem to only be able to grab the instance of the class itself, rather than the specific instance variables within the class.
Below are the two classes.
product.rb
class Product
attr_reader :name,
:price
def initialize(name, price)
#name = name
#price = price
end
end
catalogue.rb
class Catalogue
attr_reader :contents
def initialize
#contents = []
end
def cheapest
#contents.product.first
end
def <<(product)
#contents << product
end
end
The following test confirms I'm not properly extracting and sorting #name nor #price for the instance of Product that's stored in #contents in the instance of Catalogue.
catalogue_test.rb
gem 'minitest', '~> 5.2'
require 'minitest/autorun'
require 'minitest/pride'
require 'catalogue'
require 'product'
class CatalogueTest < Minitest::Test
def test_cheapest_of_one
catalogue = Catalogue.new
catalogue << Product.new('scissors', 8)
assert_equal 'scissors', catalogue.cheapest
end
end
Here is the failure:
Failure:
CatalogueTest#test_cheapest_of_one [catalogue_test.rb:16]:
--- expected
+++ actual
## -1 +1 ##
-"scissors"
+[#<Product:0xXXXXXX #name="scissors", #price=8>]
Ideally, I'd like to be able to extract and sort the product based on price across a number of instances. I realize I will need to further extend the existing code to ensure I'm grabbing the cheapest (when there is more than one object in the array), but was just trying to start with a basic function to access the elements in it.
Thus far I've tried some different methods like sort and first, however, I'm unable to go beyond the above output, [#<Product:0xXXXXXX #name="scissors", #price=8>], and drill down into the instance variables. 🤷‍♂️
What would I need to add to catalogue.rb to make test_cheapest_of_one in catalogue_test.rb pass?

Answer 1

I think your test will work with the following method definition:
def cheapest
#contents.sort_by(&:price).reverse.first.name
end
or alternatively
def cheapest
#contents.sort_by { |product| produce.price * -1 }.first.name
end
Currently you're trying calling #contents.product, which is actually calling Array#product - not what you want.

Answer 2

In your question, you ask:
What would I need to add to catalogue.rb to make test_cheapest_of_one in catalogue_test.rb pass?
But, that is the wrong question to ask! As I alluded to in my comment above, your problem is that, in your test, you are testing whether a Product is equal to a String, which can never be true, because a Product will never be equal to a String. A Product will only be equal to a Product and a String will only be equal to a String.
So, the problem is with your test, not with the production code.
If you were to modify your production code, you would have to change the cheapest method to return the name of the cheapest product. But that is wrong. It should return the cheapest product. It doesn't help you if it returns the name, because there's nothing useful you can do with the name. You cannot print out how much the cheapest product costs, for example, because you don't know what the cheapest product is, you only know what it is called.
The correct solution is to modify the test, so that it tests that the correct product is returned, not the name:
def test_cheapest_of_one
catalogue = Catalogue.new
scissors = Product.new('scissors', 8)
catalogue << scissors
assert_equal scissors, catalogue.cheapest
end