instance_method_already_implemented? (ActiveRecord::AttributeMethods::ClassMethods)

    # File lib/active_record/attribute_methods.rb, line 84
          def instance_method_already_implemented?(method_name)
            return true if method_name =~ /^id(=$|\?$|$)/
            @_defined_class_methods         ||= Set.new(ancestors.first(ancestors.index(ActiveRecord::Base)).collect! { |m| m.public_instance_methods(false) | m.private_instance_methods(false) | m.protected_instance_methods(false) }.flatten)
            @@_defined_activerecord_methods ||= Set.new(ActiveRecord::Base.public_instance_methods(false) | ActiveRecord::Base.private_instance_methods(false) | ActiveRecord::Base.protected_instance_methods(false))
            raise DangerousAttributeError, "#{method_name} is defined by ActiveRecord" if @@_defined_activerecord_methods.include?(method_name)
            @_defined_class_methods.include?(method_name)
          end