create.js.erb

alert("test" + <%= photo %>);

photos_controller.rb

  def create
    @photo = Photo.new(params[:photo])

    respond_to do |format|
      if @photo.save
        format.html { redirect_to @photo, notice: 'Photo was successfully created.' }
        format.json { render json: @photo, status: :created, location: @photo }
        format.js #added to ajax the creation of a photo and send attr back
      else
        format.html { render action: "new" }
        format.json { render json: @photo.errors, status: :unprocessable_entity }
      end
    end
  end