Extract `readonly_attribute?`

activerecord/lib/active_record/attribute_methods.rb

@@ -437,7 +437,7 @@ def attributes_with_values(attribute_names)
       def attributes_for_update(attribute_names)
         attribute_names &= self.class.column_names
         attribute_names.delete_if do |name|
-          readonly_attribute?(name)
+          self.class.readonly_attribute?(name)
         end
       end
 
@@ -460,10 +460,6 @@ def format_for_inspect(value)
         end
       end
 
-      def readonly_attribute?(name)
-        self.class.readonly_attributes.include?(name)
-      end
-
       def pk_attribute?(name)
         name == @primary_key
       end

activerecord/lib/active_record/persistence.rb

@@ -939,7 +939,7 @@ def _create_record(attribute_names = self.attribute_names)
     end
 
     def verify_readonly_attribute(name)
-      raise ActiveRecordError, "#{name} is marked as readonly" if self.class.readonly_attributes.include?(name)
+      raise ActiveRecordError, "#{name} is marked as readonly" if self.class.readonly_attribute?(name)
     end
 
     def _raise_record_not_destroyed

activerecord/lib/active_record/readonly_attributes.rb

@@ -19,6 +19,10 @@ def attr_readonly(*attributes)
       def readonly_attributes
         _attr_readonly
       end
+
+      def readonly_attribute?(name) # :nodoc:
+        _attr_readonly.include?(name)
+      end
     end
   end
 end