all_in_controller.rb

# Typical way of handling the email in the controller
class UsersController < ApplicationController
  def create
    @user = User.new params[:user]
    UserMailer.deliver_welcome(@user) if @user.save
  end
end

intention_revealing_method.rb

class UsersController < ApplicationController
  def create
    @user = User.new params[:user]
    @user.signup
  end
end

class User < ActiveRecord::Base
  def signup
    UserMailer.deliver_signup(self) if save
  end
end

observer_pattern.rb

# Using ActiveRecord::Callbacks as an observer
class UsersController < ApplicationController
  def create
    @user = User.new params[:user]
    @user.save
  end
end

class User < ActiveRecord::Base
  after_create { |new_user|
    UserMailer.deliver_welcome(new_user)
  }
end

service.rb

# Using a service to remove duplication
class UsersController < ApplicationController
  def create
    @user = User.new params[:user]
    UserService.signup @user
  end
end

class Admin::UsersController < AdminController
  def create
    @user = User.new params[:user]
    UserService.signup(@user)
  end
end

class UserService
  def self.signup(user)
    UserMailer.deliver_welcome(user) if user.save
  end
end