class Site
  include Mongoid::Document
  field :name, :type => String

  has_and_belongs_to_many :emails, inverse_of: :sites, class_name: 'Email'
  belongs_to :owner, inverse_of: :owner_of, class_name: 'Email'

  validates :emails, :presence => { :message => "error" }

end

class Email
  include Mongoid::Document
  field :address, :type => String

  has_and_belongs_to_many :sites, inverse_of: :emails, class_name: 'Site'
  has_many :owner_of, inverse_of: :owner, class_name: 'Site'

end