[Fix #1071] Fix an error occurring in the Rails/FilePath cop when Fil…

…e.join is used with a variable

.rubocop_todo.yml

@@ -29,7 +29,7 @@ InternalAffairs/OnSendWithoutOnCSend:
 # Offense count: 10
 # Configuration parameters: CountComments, CountAsOne.
 Metrics/ClassLength:
-  Max: 179
+  Max: 163
 
 # Offense count: 41
 # Configuration parameters: CountComments, CountAsOne, AllowedMethods, AllowedPatterns.

changelog/fix_an_error_occurring_in_the_rails_file_path_cop_when_file_join_is_used_with_a_variable.md

@@ -0,0 +1 @@
+* [#1071](https://github.com/rubocop/rubocop-rails/issues/1071): Fix an error occurring in the `Rails/FilePath` cop when `File.join` is used with a variable. ([@ydakuka][])

lib/rubocop/cop/rails/file_path.rb

@@ -34,7 +34,7 @@ module Rails
       #   # good
       #   Rails.root.join('app', 'models', 'goober').to_s
       #
-      class FilePath < Base
+      class FilePath < Base # rubocop:disable Metrics/ClassLength
         extend AutoCorrector
 
         include ConfigurableEnforcedStyle
@@ -98,6 +98,7 @@ def check_for_extension_after_rails_root_join_in_dstr(node)
         def check_for_file_join_with_rails_root(node)
           return unless file_join_nodes?(node)
           return unless node.arguments.any? { |e| rails_root_nodes?(e) }
+          return if node.arguments.any?(&:variable?)
 
           register_offense(node, require_to_s: true) do |corrector|
             autocorrect_file_join(corrector, node) unless node.first_argument.array_type?

spec/rubocop/cop/rails/file_path_spec.rb

@@ -322,6 +322,72 @@
         RUBY
       end
     end
+
+    context 'when using File.join with a local variable' do
+      it 'does not register an offense' do
+        expect_no_offenses(<<~RUBY)
+          default_path = '/models'
+          File.join(Rails.root, 'app', default_path)
+        RUBY
+      end
+    end
+
+    context 'when using File.join with an instance variable' do
+      it 'does not register an offense' do
+        expect_no_offenses(<<~RUBY)
+          File.join(Rails.root, 'app', @default_path)
+        RUBY
+      end
+    end
+
+    context 'when using File.join with a class variable' do
+      it 'does not register an offense' do
+        expect_no_offenses(<<~RUBY)
+          File.join(Rails.root, 'app', @@default_path)
+        RUBY
+      end
+    end
+
+    context 'when using File.join with a global variable' do
+      it 'does not register an offense' do
+        expect_no_offenses(<<~RUBY)
+          File.join(Rails.root, 'app', $default_path)
+        RUBY
+      end
+    end
+
+    context 'when using Rails.root.join with a local variable' do
+      it 'does not register an offense' do
+        expect_no_offenses(<<~RUBY)
+          default_path = '/models'
+          Rails.root.join(Rails.root, 'app', default_path)
+        RUBY
+      end
+    end
+
+    context 'when using Rails.root.join with an instance variable' do
+      it 'does not register an offense' do
+        expect_no_offenses(<<~RUBY)
+          Rails.root.join(Rails.root, 'app', @default_path)
+        RUBY
+      end
+    end
+
+    context 'when using Rails.root.join with a class variable' do
+      it 'does not register an offense' do
+        expect_no_offenses(<<~RUBY)
+          Rails.root.join(Rails.root, 'app', @@default_path)
+        RUBY
+      end
+    end
+
+    context 'when using Rails.root.join with a global variable' do
+      it 'does not register an offense' do
+        expect_no_offenses(<<~RUBY)
+          Rails.root.join(Rails.root, 'app', $default_path)
+        RUBY
+      end
+    end
   end
 
   context 'when EnforcedStyle is `arguments`' do