Multiple views in Rails

Question

I have the following problem:

In Rails I have two Objects: Categories and Items One category has many items and one item belongs to one category.

Okay. There are no problems. But now, I want to display all existing categories in a sidebar on every page in my project.

I tried to do display them like:

<div class="sidebar">
  <% @categories.each do |category| %>
    <p><%= link_to category.title, category %></p>
  <% end %>
</div>

My root controller is categories. On my starting page the code above will work without any problems.

But when I click on a category I get the following exception:

*You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.each*

Extracted source (around line #2):
2:     <% @categories.each do |category| %>

How can I fix this problem?

Sorry for my bad English!

Answer 1

You forget to set the @categories variable in the controller. You can either load the @categories in every action you need to display the sidebar or delegate the request to the view without setting an instance variable.

# in the controller

def action
  @categories = ...
end

# or in the view

<div class="sidebar">
  <% Category.all.each do |category| %>
    <p><%= link_to category.title, category %></p>
  <% end %>
</div>

If the code spans over multiple views/controllers, I would suggest you to extract the statement in a before_filter and/or a partial.