sessions_controller.rb

  def create
    @errors = []
    @user = User.authenticate(params[:username], params[:password])

    respond_to do |format|
      if @user.nil?
        @errors << t("authentication_error")

        format.html { render :new }
        format.json { render json: @errors }
      else
        session[:user_id] = @user.id

        format.html { redirect_to @user, notice: t("authenticated", nickname: @user.nickname) }
        format.json { render json: @user, location: @user }
      end
    end
  end