class ActiveFedora::Associations::Builder::BelongsTo

def self.define_validations(model, reflection)
  reflection.options[:optional] = !reflection.options.delete(:required) if reflection.options.key?(:required)
  required = if reflection.options[:optional].nil?
               model.belongs_to_required_by_default
             else
               !reflection.options[:optional]
             end
  super
  model.validates_presence_of reflection.name, message: :required if required
end