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Create your devise User model

We start by getting devise up and running and generating the User model.

rails generate devise User

Then you'll probably run rake db:migrate and follow their instructions for making sure you have a few default things in order. Okay, so far so good.

Add a role column to Users

We're going to identify the user as an admin with the help of enum. So let's generate a migration

rails generate migration add_role_to_users

Pop that sucker open and add

class AddRoleToUsers < ActiveRecord:Migration
  def change
    add_column :users, :role, :integer
    # Make sure all potentially existing users are set to just plain old users
    User.update_all role: 0
  end
end

Define the roles

In user.rb add the following

enum role: [:user, :admin]
after_initialize :set_default_role, if => :new_record?

private

  def set_default_role
    self.role ||= :user
  end

Then what?

Well, now you're in the position to create a helper. For example:

module SessionHelper

  def admin_user?
    if current_user.try(:role)
      if current_user.role == 'admin'
        true
      end
    end
  end
end

And that's cool, because you can test in your controllers. Maybe something like:

def update
  if admin_user?
    @posts = Post.all
  end
end