libactive_recordattribute_methods.rb

      def instance_method_already_implemented?(method_name)
        method_name = method_name.to_s
        return true if method_name =~ /^id(=$|\?$|$)/
        @_defined_class_methods         ||= ancestors.first(ancestors.index(ActiveRecord::Base)).sum([]) { |m| m.public_instance_methods(false) | m.private_instance_methods(false) | m.protected_instance_methods(false) }.map(&:to_s).to_set
        @@_defined_activerecord_methods ||= (ActiveRecord::Base.public_instance_methods(false) | ActiveRecord::Base.private_instance_methods(false) | ActiveRecord::Base.protected_instance_methods(false)).map(&:to_s).to_set
        raise DangerousAttributeError, "#{method_name} is defined by ActiveRecord" if @@_defined_activerecord_methods.include?(method_name)
        @_defined_class_methods.include?(method_name)
      end