app/models/mapping_version.rb

class MappingVersion < ActiveRecord::Base
  belongs_to :mapping
  belongs_to :source, class_name: 'OntologyVersion'
  belongs_to :target, class_name: 'OntologyVersion'

  attr_accessible :mapping,
                  :required_cons_status,
                  :proven_cons_status,
                  :proof_status,
                  :source,
                  :target,
                  :source_id,
                  :target_id,
                  :version_number
  before_create :increase_number

  private

  def increase_number
    self.version_number =
      if mapping.versions.current
        mapping.versions.current.version_number + 1
      else
        1
      end
  end
end