[Fix #680] Fix a false positive for Rails/ReversibleMigrationMethodDefinition

Fixes #680.

This PR fixes a false positive for `Rails/ReversibleMigrationMethodDefinition`
when using an inner class.

Author: koic

changelog/fix_a_false_positive_for_rails_reversible_migration_method_definition.md

@@ -0,0 +1 @@
+* [#680](https://github.com/rubocop/rubocop-rails/issues/680): Fix a false positive for `Rails/ReversibleMigrationMethodDefinition` when using an inner class. ([@koic][])

lib/rubocop/cop/rails/reversible_migration_method_definition.rb

@@ -49,15 +49,15 @@ class ReversibleMigrationMethodDefinition < Base
               'both an `up` and a `down` method.'
 
         def_node_matcher :change_method?, <<~PATTERN
-          [ #migration_class? `(def :change (args) _) ]
+          `(def :change (args) _)
         PATTERN
 
         def_node_matcher :up_and_down_methods?, <<~PATTERN
-          [ #migration_class? `(def :up (args) _) `(def :down (args) _) ]
+          [`(def :up (args) _) `(def :down (args) _)]
         PATTERN
 
         def on_class(node)
-          return if change_method?(node) || up_and_down_methods?(node)
+          return if !migration_class?(node) || change_method?(node) || up_and_down_methods?(node)
 
           add_offense(node)
         end

spec/rubocop/cop/rails/reversible_migration_method_definition_spec.rb

@@ -76,6 +76,19 @@ def add_users_column(column_name, null: false)
     RUBY
   end
 
+  it 'does not register an offense with an inner class' do
+    expect_no_offenses(<<~RUBY)
+      class SomeMigration < ActiveRecord::Migration[6.0]
+        class Foo
+        end
+
+        def change
+          add_column :users, :email, :text, null: false
+        end
+      end
+    RUBY
+  end
+
   it 'registers offenses correctly with any migration class' do
     expect_offense(<<~RUBY)
       class SomeMigration < ActiveRecord::Migration[5.2]