Why am I getting 'nil' is not an ActiveModel-compatible object. It must implement :to_partial_path error?

Why am I getting the following error?

nil is not an ActiveModel-compatible object. It must implement :to_partial_path.

I think the error may relate to the tutorial I'm following is using Rails 3.2 while I'm using Rails 4.

Here is the model code:

class DashboardsController < ApplicationController
  def show
    @text_shout = TextShout.new
    @photo_shout = PhotoShout.new
    @shouts = current_user.shouts
  end
end

class PhotoShoutsController < ApplicationController
  def create
    content = build_content
    shout = current_user.shouts.build(content: content)
    if shout.save
      redirect_to dashboard_path
    else
      flash.alert = "Could not shout."
      redirect_to dashboard_path
    end
  end

  private
  def build_content
    PhotoShout.new(photo_shout_parameters)
  end

  def photo_shout_parameters
    params.require(:photo_shout).permit(:image)
  end
end

Here is the view code with the error occurring on the _shout.html partial

# app/view/dashboards/show.html.erb
<%= form_for @text_shout do |form| %>
   <%= form.text_field :body, placeholder: 'Shout content here' %>
   <%= form.submit 'Shout' %>
<% end %>

<%= form_for @photo_shout do |form| %>
   <%= form.file_field :image %>
   <%= form.submit 'Shout' %>
<% end %>

<%= render @shouts %>

# app/view/shouts/_shout.html.erb
<%= div_for shout do %>
  <%= link_to shout.user.username, shout.user %>
  shouted
                                 +---------------------------------+
  <%= render shout.content %> <--| ERROR "nil' is not an Active "  |
                                 | "Model-compatible object"       |
                                 +---------------------------------+
  <%= link_to time_ago_in_words(shout.created_at), shout %>
<% end %>

# app/views/photo_shouts/_photo_shout.html.erb
<%= image_tag photo_shout.image.url(:shout) %>

The problem you are having is because you have existing records in your database that don't have content associated to them. This happens because you went from a non-polymorphic setup to a polymorphic setup. What you need to do is look for shouts that are missing content_type and content_id and remove them from the database. Once those are removed, it could be useful to add

validates_associated :content

to your Shout model to ensure data in the future doesn't end up "corrupting" your database.