Don't use instance variables in partials

One of my favorite pet peeves is instance variables in partials. Unfortuntaly, I still encounter these in actual production code.

I get it, it happens. You have a view template that needs cleaning up:

# app/views/employees/show.html.erb
...
<% @meetings.each do |meeting| %>
  A bunch of code to render a meeting
<% end %>
...

Easy enough, just extract some parts of it into a partial:

# app/views/employees/show.html.erb
...
<%= render("meetings") %>
...

# app/views/employees/_meetings.html.erb
<% @meetings.each do |meeting| %>
  A bunch of code to render a meeting
<% end %>

Now you suddenly have an instance variable in the partial. No biggie, you think, it works perfectly fine and that instance variable is already defined.

Why is this bad?

You’ve just coupled the partial to - not just the view - but the controller action, which is where the instance variable is defined.

If you ever need to use that partial in a different context it’ll break. If you remove the @meetings variable from the action (which isn’t unlikely because it’s obviously not being used in the view, so why even create it?) it’ll break. Boom, NoMethodError: undefined method 'each' for nil.

Instead, use locals. Even better, use strict locals. Or perhaps even better, ViewComponents, but that’s out of scope for this article.

Locals

If we instead rely on a local variable in the partial, the error would’ve been undefined method or variable, meetings, which is much more direct understandable:

# app/views/employees/_meeting.html.erb
<% meetings.each do |meeting| %>
  A bunch of code to render a meeting
<% end %>

We can then use the partial like this:

<% render("meetings", :meetings => @meetings) %>

While this is a bit more typing up front, it makes it painfully obvious what data the partial needs.

Strict locals

To make it even more explicit what locals a partial needs, we can configure the partial to use strict locals. Effectively document the “method signature” for a partial:

<%# locals (meetings:) %>
<% meetings.each do |meeting| %>
  A bunch of code to render a meeting
<% end %>

If we then try to render the partial without passing the :meetings local we’d get a ArgumentError: missing local: :meetings - again, much more explicit, understandable and actionable.

Don’t take my word for it

If you need more arguments against using instance variables in partials, just listen to Andy Croll.