Rails - Validate Nested Attributes Uniqueness with scope parent of parent

I have a problem with the scoped uniqueness validation in Rails for nested attributes with a parent of parent.

Background

I have a rails 4 application with 3 models :

#app/models/account.rb
class Account < ActiveRecord::Base
  has_many :contacts, dependent: :destroy
end

#app/models/contact.rb
class Contact < ActiveRecord::Base
  belongs_to :account
  has_many :email_addresses, dependent: :destroy, validate: :true, inverse_of: :contact
  accepts_nested_attributes_for :email_addresses,allow_destroy: true
  validates :email_addresses, presence: true
end

#app/models/email_address.rb
class EmailAddress  < ActiveRecord::Base
  belongs_to :contact, inverse_of: :email_addresses

  validates :label, presence: true
  validates :contact, presence: true
  validates :email, uniqueness: true, presence: true
  validates_email_format_of :email
end

Issue

I want make a scope, so as to make sure the attribute :email of the model EmailAddress is unique at the Account Level (Account is parent of Contact, which is itself parent of EmailAddress).

As suggested at http://guides.rubyonrails.org/active_record_validations.html, I tried :

 class EmailAddress  < ActiveRecord::Base
  belongs_to :contact, inverse_of: :email_addresses

  validates :label, presence: true
  validates :contact, presence: true
  validates :email, presence: true, uniqueness: { scope: :account, 
                    message: "This contact email is already taken" }
  validates_email_format_of :email
 end

This raises the error "column email_addresses.account does not exist" What should I do ?

Thanks for you help !

A better option in terms of performances is described below. It is tested and works just fine.

Why?

Mapping emails can consume a lot of ressources when a lot of emails are at stake, so its better to perform the scope directly with the database.

How?

Cashing the account_id in the EmailAddress model and performing a before validation method.

1) Create a migration :

change_table :email_addresses do |t|
  t.references :account, index: true
end
add_index :email_addresses, [:account_id, :email], unique: true

2) Migrate

3) Update the EmailAddress model

#app/models/email_address.rb

class EmailAddress < ActiveRecord::Base
  belongs_to :contact, inverse_of: :email_addresses
  belongs_to :account

  validates :label, presence: true
  validates :contact, presence: true
  validates_email_format_of :email
  validates_uniqueness_of :email, allow_blank: false, scope: :account

  before_validation do
    self.account = contact.account if contact
  end

end