Don't use instance variables in partials

Rails’ view architecture is a flexible and magical place. Perhaps too much so. Some examples of ‘view magic’ include:

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• Instance variables defined in controllers are magically available in view templates.
• Inferred template names.
• A global namespace of helpers.

For some it’s too much, but most of us accept the “magic” and muddle through any strangeness.

Instead of…

…using the global variables from your template inside your partials…

album/show.html.erb

...
<% @album.songs.each do |song| %>
  <h2><%= song.name %></h2>
  <%= render "character", collection: song.characters, as: :character %>
<% end %>
...

song/_character.html.erb

...
<p><%= character.name %></p>
<div class="popup">
  Other Songs: <%= character.features_on_tracks(@album.songs).to_sentence %>
</div>
...

Always…

…pass variables into your partials, as local variables, so they don’t expect any global state.

album/show.html.erb

...
<% @album.songs.each do |song| %>
  <h2><%= song.name %></h2>
  <%= render "character", collection: song.characters,
                          as: :character,
                          locals: { songs: @album.songs } %>
<% end %>
...

song/_character.html.erb

...
<p><%= character.name %></p>
<div class="popup">
  Other Songs: <%= character.features_on_tracks(songs).to_sentence %>
</div>
...

But why?

It’s too easy to presume that certain instance variables, in this case @album, are available in the context your partial is called in.

When you first extract a partial, often for organisational reasons, it is typically used in only one place. When you reuse the partial somewhere else in your application, the controller action in which it is eventually called must correctly assign the same instance variable from the original controller action.

As your application becomes more complex, a view might contain several partials—each expecting their own instance variables—so you’ll need to assign several instance variables in the controller action. The context of these assignments will be a long way away from the nested partial in which they’re used.

If you rely on the “Rails magic” early on, you often pay in complexity later on. This is a pattern of many of the issues that people have with Rails’ HTML rendering environment. If you avoid using instance variables in your partials, they become simpler to reuse, even as your application grows in complexity

A little help

You can find lines in view partials which reference an instance variable by using this command from the root of your application in your terminal:

grep -rEHn "\@\w+" --include "_*.erb" app/views

Why not?

It might feel easier to leave any instance variables in place and it can also feel redundant to write the additional code needed in order to define the partial’s variables in the call to render. However, not making these changes will bite you in the end.

It’s certainly never a good idea to create an instance variable to share it between partials.

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