sinatra-formkeeper combination-diff

app.rb

require 'sinatra/base'
require 'sinatra/formkeeper'

class MainApp < Sinatra::Base
  register Sinatra::FormKeeper
  post '/input' do
    form do
      filters :strip
      field :username, :present => true, :length => 4..12
      field :old_password, :present => true, :length => 8..16
      field :new_password, :present => true, :length => 8..16
      diff :different_password, ['old_password', 'new_password']
    end
    if form.failed?
      'input failed'
    else
      'input success ' + form[:username]
    end
  end
end

app_spec.rb

require File.dirname(__FILE__) + '/spec_helper'

describe 'Main' do
  include Rack::Test::Methods
  context 'post to /input' do
    context '(old|new)_password are different' do
      valid_param = {
        username: 'valid_name',
        old_password: 'password_old',
        new_password: 'password_new',
      }
      it 'success response' do
        post '/input', valid_param
        expect(last_response.body).to eq 'input success valid_name'
      end
    end

    context '(old|new)_password are same' do
      invalid_param = {
        username: 'valid_name',
        old_password: ' password_same',
        new_password: 'password_same ',
      }
      it 'failed response' do
        post '/input', invalid_param
        expect(last_response.body).to eq 'input failed'
      end
    end
  end
end