BathRuby 2015 - Nothing is Something by Sandi Metz 前半のメモ

動画
https://www.youtube.com/watch?v=9lv2lBq6x4A&feature=youtu.be

missingno15さんがこちらに書いたコードを自分のメモとして記述｡
https://gist.github.com/missingno15/4bc7efabe7a45ff895fd

require 'pry'

class Animal
  DATABASE = [
    { id: 1, name: "Mockingbird" },
    { id: 2, name: "Pheasant" },
    { id: 3, name: "Duck" },

    { id: 4, name: "Platypus" },
    { id: 5, name: "Penguin" },
    { id: 6, name: "Peacock" },
    { id: 7, name: "Hummingbird" },
    { id: 8, name: "Owl" }
  ]

  attr_accessor :id, :name

  def self.find(id)
    target_data = DATABASE.find { |data| data[:id] == id }

    if target_data
      self.new(target_data)
    end
  end

  def initialize(attrs)
    @id = attrs.fetch(:id)
    @name = attrs.fetch(:name)
  end
end


# Now we have a list of ids that we want to query

ids = [4, 8]

# Let's map this

birds = ids.map { |id| Animal.find(id) }
p birds  # => [{:id=>4, :name=>"Platypus"}, {:id=>8, :name=>"Owl"}]

# Let's now add an id that doesn't exist in our database to our array of ids
# Now when we run map on the ids, we should get nil in the resulting array


ids << 48
birds = ids.map { |id| Animal.find(id) }
p birds  # => [{:id=>4, :name=>"Platypus"}, {:id=>8, :name=>"Owl"}, nil]

# However, let's say we don't want it to return 'nil', we want it to be 
# descriptive and return 'no animal' instead when we call #name on each object
# With our current implementation, if you iterate through each element 
# in the array, you'll get:

# birds.each { |bird| bird.name }
# => undefined method `name' for nil:NilClass (NoMethodError)

# Let's say we can't change Animal#find because it belonged to an 
# external framework so we have no control whatsoever on it

# You could do something like this:

birds.each { |bird| bird ? (puts bird.name) : (puts "no animal") }

# However, we can do better
# Let's use the Null Object Pattern to substitute for our missing values

class MissingAnimal
  def name
    "no animal"
  end
end

birds = ids.map { |id| Animal.find(id) || MissingAnimal.new }

# Now when when we call #name on each object

birds.each { |bird| puts bird.name }


# #=> Platypus
#     Owl
#     No animal


# However, Sandi Metz makes the point that everytime you want to do this, we will be calling on 
# twice the amount of objects. Her solution is wrapping up the Null Object into another class 
# which also handles the API of the thing that we have no control over.

class GuaranteedAnimal
  def self.find(id)
    Animal.find(id) || MissingAnimal.new
  end
end


# Now in the rest of the areas of your app, you can now change this:

birds = ids.map { |id| Animal.find(id) || MissingAnimal.new }

# to this↓

birds = ids.map { |id| GuaranteedAnimal.find(id) }

birds.each { |bird| puts bird.name  }
# => Platypus
#    Owl
#    No animal