RSpecカスタムマッチャでデータベースのNOT NULL制約をテストする

describe Person do
  subject { FactoryGirl.create(:person) }
  it { should have_not_null_constraint_on(:title) }
end

RSpec::Matchers.define :have_not_null_constraint_on do |field|
  match do |model|
    model.send("#{field}=", nil)
    begin
      model.save!(validate: false)
      false
    rescue ActiveRecord::StatementInvalid
      true
    end
  end

  description { "have NOT NULL constraint on #{field}" }
  failure_message { "expected to have NOT NULL constraint on #{field}, but not" }
end