RSpecカスタムマッチャでデータベースのUNIQUE制約をテストする

describe Person do
  it { should have_not_null_constraint_on(:name) }
end

RSpec::Matchers.define :have_unique_constraint_on do |*fields|
  match do |model|
    name = model.class.table_name.singularize
    record = FactoryGirl.create(name)
    other_record = FactoryGirl.build(name)

    fields.each do |field|
      other_record.send("#{field}=", record.send(field))
    end

    begin
      other_record.save!(validate: false)
      false
    rescue ActiveRecord::RecordNotUnique
      true
    end
  end

  description { "have UNIQUE constraint on #{fields.join(", ")}" }
  failure_message { "expected to have UNIQUE constraint on #{fields.join(", ")}, but not" }
end