Show Rails errors, notices and devise messages in one place - ruby-on-rails

Question

I have my application.html.erb file where this code located:
<% if notice %>
<p class="notice"><%= notice %></p>
<% elsif alert %>
<p class="alert"><%= alert %></p>
<% end %>
<%= devise_error_messages! %>
All is great, but form validations are made by using (as usual), this:
<%= form_for #project do |f| %>
<% if #project.errors.any? %>
<h2><%= pluralize(#project.errors.count, "error") %> prevented this project from saving:</h2>
<ul>
<% #project.errors.full_messages.each do |msg| %>
<li><%= msg %></li>
<% end %>
</ul>
<% end %>
...
<% end %>
How to include them into main app page & show them always in top?

Answer 1

How to include them into main app page & show them always in top?
This is deeper than just adding a partial (as Umang Raghuvanshi suggested).
The problem is that the errors method is part of an #object, which means you have to make sure you're able to access this object whenever you want to output the errors in application.html.erb.
Partials should never be dependent on #instance_variables - they're meant to be loaded in any part of your app. You can pass local variables to them, allowing you to send predefined data as you need.
In your case, I would use a partial but populate it with a content_for:
--
This is what I'd do:
#app/views/layouts/application.html.erb
<%= render partial: "shared/errors" %>
#app/views/shared/_errors.html.erb
<%= content_tag :div, class: "errors" do %>
<% if errors.try?(:any?) %>
<% errors.full_messages.each do |msg| %>
<%= content_tag :li, msg %>
<% end %>
<% else %>
<% flash = %w(notice error alert) %>
<% flash.each do |f| %>
<%= content_tag :div, class: f, flash => [f.to_sym] if flash[f.to_sym] %>
<% end %>
<%= devise_error_messages! %>
<% end %>
<% end %>
#app/views/projects/new.html.erb
<%= form_for #project do |f| %>
<%= render "shared/errors", locals: {errors: #project.errors} %>
<% end %>
This will allow you to style the .errors div to be absolutely position at the top:
JSFiddle
#app/assets/stylesheets/application.css
.errors {
display: inline-block;
width: 50%;
position: absolute;
top: 5%;
}

Answer 2

Extract the code into a partial and include it into your application.html.erb.
_your_partial_name.erb
<% if #project.errors.any? %>
<h2><%= pluralize(#project.errors.count, "error") %> prevented this project from saving:</h2>
<ul>
<% #project.errors.full_messages.each do |msg| %>
<li><%= msg %></li>
<% end %>
</ul>
<% end %>
application.html.erb
<%= render partial: 'your-partial-name' %>
<% if notice %>
<p class="notice"><%= notice %></p>
<% elsif alert %>
<p class="alert"><%= alert %></p>
<% end %>
<%= devise_error_messages! %>
You could also try using flash.