when #user is blank, how to make this if statement work? - ruby-on-rails

Question

I have a header partial linked to my application.html.erb that looks like this:
<header class="unselectable">
<h2 class="float_left">
<% if #user.try(:errors).present? %>
<%= render 'shared/error_messages' %>
<% else %>
<%= #title %>
<% end %>
</h2>
<nav class="round">
<ul>
<% if logged_in? %>
<li><%= link_to "Home", current_user %></li>
<li><%= link_to "Settings", edit_user_path %></li>
<li><%= link_to "Log out", logout_path %></li>
<% else %>
<li><%= link_to "Log in", login_path %></li>
<% end %>
</ul>
</nav>
</header>
This is all well and good unless the page that loads doesn't have an #user variable (such as an about or logout page) in which case i get this:
undefined method `errors' for nil:NilClass
How can I make this work? I tried changing the logic to render the title unless #user.errors.any?but that didn't work either. I'm sure this is a simple fix but I can't figure it out!
EDIT added the fixes suggested (updated in the header partial above) and now get this error:
No route matches {:action=>"edit", :controller=>"users"} which seems to be coming from the edit_user_path

Answer 1

You can use the method .try(:something):
<% if #user.try(:errors).present? %>
<%= render 'shared/error_messages' %>
<% else %>
<%= #title %>
<% end %>
If #user is nil, the .try(:errors) will not raise an error.
The .present? method works for nil too:
.
>> nil.present?
#=> false
>> false.present?
#=> false
>> [].present?
#=> false
>> ''.present?
#=> false
>> 'bonjour'.present?
#=> true
>> ['bonjour'].present?
#=> true
.present? is a combination of .nil? AND .empty?
.present? is actually the opposite result of .blank?

Answer 2

I highly question the need for #user in your partial which is rendered in your application layout, hence its need in every page of your application. I argue that this is not good design at all because now you're relying on a global variable in all views of your application.
I think what you really mean to use is the flash. In which case you want something like this in application.html.erb.
<% flash.each do |key, value| %>
<%= content_tag :div, value, class: key %>
<% end %>
This should be set in the appropriate controller action before it's view is rendered so that the error message displys according to the request that was just made.
If your error messages come from your models, then this should be part of what actually generates these error messages. Typically this is a call to either create or update actions in the controller. In which case you should have the error_messages partial rendered with the form when your validations do not pass and the form is rendered again with the model object.
<%= form_for #user do |f| %>
<%= render 'shared/error_messages', :object => f.object %>
<!-- and so on -->
<% end %>
This way you can be confident that the #user object is always available for the partial to render without any errors since we're explicitly passing the object to the partial itself, and the partial is being rendered with the correct context. Using #users in your partial itself is the equivalent of using a global variable, hence the entire application relying on that global variable to exist.
The #user object is now accessed with a local variable in the partial as object (or whatever your decide to end up naming it).
<% object.errors.full_messages.each do |message| %>
<li>* <%= message %></li>
<% end %>

Answer 3

You can reformulate to like this:
<header>
<h2 class="float_left">
<% if #user.try(:errors).try(:any?) %>
<%= render 'shared/error_messages' %>
<% else %>
<%= #title %>
<% end %>
</h2>
...
</header>
Or add errors_any? to model:
class User
def errors_any?
self.try(:errors).try(:any?)
end
end
And to this:
<header>
<h2 class="float_left">
<% if #user.try(:errors_any?) %>
<%= render 'shared/error_messages' %>
<% else %>
<%= #title %>
<% end %>
</h2>
...
</header>