Undefined method error on a nested resource index page

As a follow up to this earlier question, which has been solved

I have made a beginning with a rails app where I can create Collections. Each Collections is able to have multiple Photos. I can now create these Collections and Photos. But whenever I try to visit /collections/1/photos it has a problem with this line in my photos index

undefined method `photo_path' for #<#<Class:0x007fb8c40f1b38>:0x007fb8c88673a0>

<td><%= link_to 'Show', photo %></td>

photos_controller.rb

class PhotosController < ApplicationController
  before_action :set_photo, only: [:show, :edit, :update, :destroy]

  # GET /photos
  # GET /photos.json
  def index
    @photos = Photo.all
  end

  # GET /photos/1
  # GET /photos/1.json
  def show
  end

  # GET /photos/new
  def new
    @photo = Photo.new
  end

  # GET /photos/1/edit
  def edit
  end

  # POST /photos
  # POST /photos.json
  def create
    @photo = Photo.new(photo_params)

    respond_to do |format|
      if @photo.save
        redirect_to @photo
        format.html { redirect_to @photo, notice: 'Photo was successfully created.' }
        format.json { render :show, status: :created, location: @photo }
      else
        format.html { render :new }
        format.json { render json: @photo.errors, status: :unprocessable_entity }
      end
    end
  end

  # PATCH/PUT /photos/1
  # PATCH/PUT /photos/1.json
  def update
    respond_to do |format|
      if @photo.update(photo_params)
        format.html { redirect_to @photo, notice: 'Photo was successfully updated.' }
        format.json { render :show, status: :ok, location: @photo }
      else
        format.html { render :edit }
        format.json { render json: @photo.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /photos/1
  # DELETE /photos/1.json
  def destroy
    @photo.destroy
    respond_to do |format|
      format.html { redirect_to photos_url, notice: 'Photo was successfully destroyed.' }
      format.json { head :no_content }
    end
  end

  private
    # Use callbacks to share common setup or constraints between actions.
    def set_photo
      @photo = Photo.find(params[:id])
    end

    # Never trust parameters from the scary internet, only allow the white list through.
    def photo_params
      params.require(:photo).permit(:name, :collection_id)
    end
end

routes.rb

Rails.application.routes.draw do
  resources :collections do
    resources :photos
  end
end

/photos/index.html

<p id="notice"><%= notice %></p>

<h1>Photos</h1>

<table>
  <thead>
    <tr>
      <th colspan="3"></th>
    </tr>
  </thead>

  <tbody>
    <% @photos.each do |photo| %>
      <tr>
        <td><%= photo.name %></td>
        <td><%= link_to 'Show', photo %></td>
        <td><%= link_to 'Edit', edit_photo_path(photo) %></td>
        <td><%= link_to 'Destroy', photo, method: :delete, data: { confirm: 'Are you sure?' } %></td>
      </tr>
    <% end %>
  </tbody>
</table>

<br>

<%= link_to 'New Photo', new_collection_photo_path %>

collection.rb

class Collection < ApplicationRecord
  has_many :photos
end

photo.rb

class Photo < ApplicationRecord
  belongs_to :collection
end

I use Rails 5.0.0.1 and ruby 2.3.0