Customizing devise views and controllers

app/models/account.rb

@@ -1,6 +1,7 @@
 class Account < ActiveRecord::Base
   # Local users
   has_one :user, inverse_of: :account
+  validates :username, uniqueness: { scope: :domain }
 
   # Avatar upload
   attr_reader :avatar_remote_url

app/models/user.rb

@@ -2,6 +2,7 @@ class User < ActiveRecord::Base
   devise :database_authenticatable, :registerable, :recoverable, :rememberable, :trackable, :validatable
 
   belongs_to :account, inverse_of: :user
+  accepts_nested_attributes_for :account
 
   validates :account, presence: true
 end