The problem

Using multiple namespaces will require to have also namespaces views. For exemple, if you have an Admin namespace, with a Admin::ProjectsController with a show action that also exists in non namespaced ProjecsController, you cannot render views/projects/show.html.erv from Admin::ProjecsController.

What we want to do

The researched behavior is to try to render the views/admin/projects/show.html.erb, and, if it doesn't exist, fallback to views/projects/show.html.erb

The solution

Use a ViewResolver in /whateveryouwant. I choosed to put it in lib/resolvers/admin_views_resolver.rb :

class Resolvers::AdminViewsResolver < ::ActionView::FileSystemResolver
  def initialize
    super('app/views')
  end

  def find_templates(name, prefix, partial, details)
    prefix = prefix.sub(/^admin\//, '')
    super(name, prefix, partial, details)
  end
end

Then make it autoload in application/config.rb :

# Add lib directory in autoload_paths
config.autoload_paths << File.join(config.root, 'lib')

And finally, instanciate it in our(s) controller(s) :

class Admin::BaseController < ApplicationController
  append_view_path Resolvers::AdminViewsResolver.new
end

What's happening here ?

We're telling to Rails to use a this view resolver for our Admin::BaseController, which is the parent controller for all our controllers under Admin namespace. This will not break anything, because we call super('app/views') in the initialize function of the resolver. So, Rails will try to find the views/admin/projects/show.html.erb, but then, only if there is no such view, will call the find_templates method from the resolver, which trim admin from prefix variable (the prefix variable contains the original path to the template).