Building a collection with Enumerable

For our new awesome projet, we're gonna display a collection of kittens.
Each kitten is defined by its name, age, and its cuteness. Cuteness being a
number between 0 and 100.

Let's say we have our collection of kittens which looks like something like
this:

# let's mock something responsible for generating a collection of resultsSearch = Struct.new(:results)# Basic representation of a kittyKitty = Struct.new(:name, :age, :cuteness)# Results setsearch = Search.new([ Kitty.new("darwin", 13, 42), Kitty.new(:kitty, 37, 97), Kitty.new("maru", 8, 64)])# Let's iteratesearch.results.each do |kitty| p kitty.name, kitty.age, kitty.cutenessend

Ok, this is a perfectly working search feature. We find some results, and
display them to the user.

The product owner of this kitten application would like to display the total
cuteness of this result set and the average cuteness.

search.results.map(&:cuteness).inject(:+).tap do |cuteness| p cuteness p cuteness / search.results.sizeend

Again, this is working. Yet, something's still bugging me. To implement our
awesome features, we make several calls to search.results, which is an
internal object of the library. It is not our domain code.

Enumerable

Let's build our own private collection of kittens. An object with this API:

Enumerable provides us with a
module that implements methods like map, find, any?… The king of
methods which make using collection in Ruby such a breeze.

So, we need a Kitten class which includes Enumerable and takes an Array
as input.

A few words about this basic class declaration. results is set as private
because we do not want it to be directly accessed, everything must use the same
api, aka the Kitten class itself.

This class accepts as input an object or a collection of objects thanks to the
Array method called in the initialize.

Yet, this code does not work. To comply to the Enumerable contract, we need
to define a each method. This is the method used behind the scenes by
Enumerable to provide other methods like map.

class Kitten include Enumerable attr_reader :results private :results def initialize(results) @results = Array(results) end def each results.each {|item| yield item } endendKitten.new([Kitty.new("ohai", 42, 100), Kitty.new("kitty", 37, 97)]).each do |kitty| p kitty.nameend# => "ohai"# => "kitty"

Now, we need to add total_cuteness and average_cuteness.

class Kitten include Enumerable attr_reader :results private :results def initialize(results) @results = Array(results) end def each results.each {|item| yield item } end def total_cuteness @total_cuteness ||= sum(&:cuteness) end def sum(&block) results.map(&block).inject(:+) end def average_cuteness total_cuteness / count endend

Now, let' inspect our brand new Kitten collection.

kitten = Kitten.new([Kitty.new("ohai", 42, 100), Kitty.new("kitty", 37, 97)])p kitten.class# => Kittenp kitten.to_a.class# => Arrayp kitten.total_cuteness# => 197p kitten.average_cuteness# => 98p kitten.count# => 2p kitten.sum(&:age)# => 79

The to_a method creates a new Array based on items present in the
collection. So, to_a returns an instance of Array, it is not a Kitten
instance anymore.

This gives us a nice interface around a collection. We can easily add useful
methods like total_cuteness or methods to comply with the
Kaminari pagination interface.

Actually, there's nothing new here. This is one of the most common patterns in
the ruby ecosystem, but lots of rails developers use this pattern in libraries
like ActiveRecord everyday without noticing that has_many associations are
not really simple instances of Array.

Original Article:

Building a collection with Enumerable