Rails: route helpers for nested resources - ruby-on-rails

Question

I have nested resources as below:
resources :categories do
resources :products
end
According to the Rails Guides,
You can also use url_for with a set of objects, and Rails will automatically determine which route you want:
<%= link_to 'Ad details', url_for([#magazine, #ad]) %>
In this case, Rails will see that #magazine is a Magazine and #ad is an Ad and will therefore use the magazine_ad_path helper. In helpers like link_to, you can specify just the object in place of the full url_for call:
<%= link_to 'Ad details', [#magazine, #ad] %>
For other actions, you just need to insert the action name as the first element of the array:
<%= link_to 'Edit Ad', [:edit, #magazine, #ad] %>
In my case, I have the following code which was fully functional:
<% #products.each do |product| %>
<tr>
<td><%= product.name %></td>
<td><%= link_to 'Show', category_product_path(product, category_id: product.category_id) %></td>
<td><%= link_to 'Edit', edit_category_product_path(product, category_id: product.category_id) %></td>
<td><%= link_to 'Destroy', category_product_path(product, category_id: product.category_id), method: :delete, data: { confirm: 'Are you sure?' } %></td>
</tr>
<% end %>
Obviously it is a little too verbose and I wanted to shorten it using the trick mentioned above from rails guides.
But if I changed the Show and Edit link as follows:
<% #products.each do |product| %>
<tr>
<td><%= product.name %></td>
<td><%= link_to 'Show', [product, product.category_id] %></td>
<td><%= link_to 'Edit', [:edit, product, product.category_id] %></td>
<td><%= link_to 'Destroy', category_product_path(product, category_id: product.category_id), method: :delete, data: { confirm: 'Are you sure?' } %></td>
</tr>
<% end %>
Neither of them works any more, and the pages complains the same thing:
NoMethodError in Products#index
Showing /root/Projects/foo/app/views/products/index.html.erb where line #16 raised:
undefined method `persisted?' for 3:Fixnum
What did I miss?

Answer 1

The way Rails is 'automagically' knowing which path to use is by inspecting the objects you pass for their classes, and then looking for a controller whose name matches. So you need to make sure that what you are passing to the link_to helper is the actual model object, and not something like category_id which is just a fixnum and therefore has no associated controller.
<% #products.each do |product| %>
<tr>
<td><%= product.name %></td>
<td><%= link_to 'Show', [product.category, product] %></td>
<td><%= link_to 'Edit', [:edit, product.category, product] %></td>
<td><%= link_to 'Destroy', [product.category, product], method: :delete, data: { confirm: 'Are you sure?' } %></td>
</tr>
<% end %>

Answer 2

I'm guessing the offending line is one of these:
<td><%= link_to 'Show', [product, product.category_id] %></td>
<td><%= link_to 'Edit', [:edit, product, product.category_id] %></td>
The product.category_id is a Fixnum and the routing can't know that a random number is supposed to map to category_id.
Use the previous URLs you had, they're more readable.