additional_domain.rb

class AdditionalDomain < AR::Base
  belongs_to :report
  belongs_to :domain
  validates :report, presence: true
  validates :domain, presence: true
end

domain.rb

class Domain < AR::Base
end

report.rb

class Report < AR::Base
  belongs_to :primary_domain, class_name: "Domain"
  has_many :additional_domains
  has_many :domains, through: :additional_domains
  validates :primary_domain, presence: true # checks object is on the association, but also allows new records
  validates :primary_domain_id, presence: true # checks numeric value is set, but this could be a non existing value
end