Only consider migration classes for MigrationClassName

changelog/fix_only_consider_migration_classes_for_migration_class_name.md

@@ -0,0 +1 @@
+* [#657](https://github.com/rubocop/rubocop-rails/pull/657): Only consider migration classes for `Rails/MigrationClassName`. ([@sunny][])

lib/rubocop/cop/rails/migration_class_name.rb

@@ -20,10 +20,13 @@ module Rails
       #
       class MigrationClassName < Base
         extend AutoCorrector
+        include MigrationsHelper
 
         MSG = 'Replace with `%<corrected_class_name>s` that matches the file name.'
 
         def on_class(node)
+          return if in_migration?(node)
+
           snake_class_name = to_snakecase(node.identifier.source)
 
           return if snake_class_name == basename_without_timestamp

spec/rubocop/cop/rails/migration_class_name_spec.rb

@@ -10,6 +10,17 @@ class CreateUsers < ActiveRecord::Migration[7.0]
         end
       RUBY
     end
+
+    context 'when defining another class' do
+      it 'does not register an offense' do
+        expect_no_offenses(<<~RUBY, filename)
+          class CreateUsers < ActiveRecord::Migration[7.0]
+            class Article < ActiveRecord::Base
+            end
+          end
+        RUBY
+      end
+    end
   end
 
   context 'when the class name does not match its file name' do