[Fix #664] Fix a false positive for Rails/MigrationClassName cop

Fixes #664.

This PR fixes a false positive for `Rails/MigrationClassName` when
`ActiveSupport::Inflector` is applied to the class name and the case
is different.

Occasionally user want to treat a word like `Oauth` as` OAuth`.
This PR is case insensitive for comparisons between migration class name
and filename. It's may not a big deal because it's based on filename.

Author: koic

changelog/fix_a_false_positive_for_rails_migration_class_name_cop.md

@@ -0,0 +1 @@
+* [#664](https://github.com/rubocop/rubocop-rails/issues/664): Fix a false positive for `Rails/MigrationClassName` when `ActiveSupport::Inflector` is applied to the class name and the case is different. ([@koic][])

lib/rubocop/cop/rails/migration_class_name.rb

@@ -22,30 +22,30 @@ class MigrationClassName < Base
         extend AutoCorrector
         include MigrationsHelper
 
-        MSG = 'Replace with `%<corrected_class_name>s` that matches the file name.'
+        MSG = 'Replace with `%<camelized_basename>s` that matches the file name.'
 
         def on_class(node)
           return if in_migration?(node)
 
-          snake_class_name = to_snakecase(node.identifier.source)
+          basename = basename_without_timestamp_and_suffix(processed_source.file_path)
 
-          basename = basename_without_timestamp_and_suffix
-          return if snake_class_name == basename
+          class_identifier = node.identifier
+          camelized_basename = camelize(basename)
+          return if class_identifier.source.casecmp(camelized_basename).zero?
 
-          corrected_class_name = to_camelcase(basename)
-          message = format(MSG, corrected_class_name: corrected_class_name)
+          message = format(MSG, camelized_basename: camelized_basename)
 
-          add_offense(node.identifier, message: message) do |corrector|
-            corrector.replace(node.identifier, corrected_class_name)
+          add_offense(class_identifier, message: message) do |corrector|
+            corrector.replace(class_identifier, camelized_basename)
           end
         end
 
         private
 
-        def basename_without_timestamp_and_suffix
-          filepath = processed_source.file_path
+        def basename_without_timestamp_and_suffix(filepath)
           basename = File.basename(filepath, '.rb')
           basename = remove_gem_suffix(basename)
+
           basename.sub(/\A\d+_/, '')
         end
 
@@ -54,17 +54,9 @@ def remove_gem_suffix(file_name)
           file_name.sub(/\..+\z/, '')
         end
 
-        def to_camelcase(word)
+        def camelize(word)
           word.split('_').map(&:capitalize).join
         end
-
-        def to_snakecase(word)
-          word
-            .gsub(/([A-Z\d]+)([A-Z][a-z])/, '\1_\2')
-            .gsub(/([a-z\d])([A-Z])/, '\1_\2')
-            .tr('-', '_')
-            .downcase
-        end
       end
     end
   end

spec/rubocop/cop/rails/migration_class_name_spec.rb

@@ -48,4 +48,23 @@ class AddBlobs < ActiveRecord::Migration[7.0]
       RUBY
     end
   end
+
+  #
+  # When `OAuth` is applied instead of `Oauth` for `oauth`.
+  #
+  # # config/initializers/inflections.rb
+  # ActiveSupport::Inflector.inflections(:en) do |inflect|
+  #   inflect.acronym 'OAuth'
+  # end
+  #
+  context 'when `ActiveSupport::Inflector` is applied to the class name and the case is different' do
+    let(:filename) { 'db/migrate/20210623095243_remove_unused_oauth_scope_grants.rb' }
+
+    it 'does not register an offense' do
+      expect_no_offenses(<<~RUBY, filename)
+        class RemoveUnusedOAuthScopeGrants < ActiveRecord::Migration[7.0]
+        end
+      RUBY
+    end
+  end
 end