【Rspec】バリデーションinclusionのテストはallow_valueを使おう

Bookモデルに下記のバリデーションを定義し、
validate_inclusion_ofでバリデーションのテストをしようとするとテストが落ちる。

app/models/book.rb

class Book < ApplicationRecord
...
  validates :read, inclusion: { in: [true, false] } 
end

spec/models/book_spec.rb

it {is_expected.to validate_inclusion_of(:read).in_array([true, false])}

validate_inclusion_ofのソースにヒントがあった。

We discourage using `validate_inclusion_of` with boolean columns. In
fact, there is never a case where a boolean column will be anything but
true, false, or nil, as ActiveRecord will type-cast an incoming value to
one of these three values. That means there isn't any way we can refute
this logic in a test. Hence, this will produce a warning:

     it do
       should validate_inclusion_of(:imported).
       in_array([true, false])
     end

The only case where `validate_inclusion_of` *could* be appropriate is
for ensuring that a boolean column accepts nil, but we recommend
using `allow_value` instead, like this:

     it { should allow_value(nil).for(:imported) }

代替案の通りにテストを書くとグリーンになる！

spec/models/book_spec.rb

it {is_expected.to allow_value(true).for(:read)}
it {is_expected.to allow_value(false).for(:read)}
it {is_expected.not_to allow_value(nil).for(:read)}