# please improve this rails action,
# if you'd like to put some code in another file please state so:
# authentication API
before_action :set_user, only [:show, :edit, :update, :destroy]
def auth
  success = ->(user) { render json: user }
  error = -> { render json: { errors: ["wrong username or password"] }, status: :unauthorized }

  UseCase::User::CheckPassword.new.call(user, params, success: success, failure: error)
end
private
def set_user
  @user = repo.find(params[:username])
end

def repo
  @repo ||= UserRepository.new(params)
end

#app/repositories/user_repository.rb
class UserRepository
  def new_entity(attrs = nil)
    User.new(attrs)
  end
  def find(attr)
    (attr.is_a?(String)) ? User.find_by_username(attr) : User.find(attr)
  end
  def check_password(attr)
  end
end

# app/use_cases/use_case/base.rb
module UseCase
  class Base
    attr_reader :repository

    def initialize(repo=nil)
      @repository = repo
    end
  end
end

# app/use_cases/use_case/user/check_password.rb
module UseCase
  module User
    class CheckPassword < UseCase::Base
      def call(user, attrs, callbacks)
        user.check_password(attrs[:password]) ? callbacks[:success].call(user) : callbacks[:failure].call()
      end
    end
  end
end