Ignore Rails.root interpolation at end of string

CHANGELOG.md

@@ -13,6 +13,7 @@
 * [#12](https://github.com/rubocop-hq/rubocop-rails/issues/12): Fix a false positive for `Rails/SkipsModelValidations` when passing a boolean literal to `touch`. ([@eugeneius][])
 * [#238](https://github.com/rubocop-hq/rubocop-rails/issues/238): Fix auto correction for `Rails/IndexBy` when the `.to_h` invocation is separated in multiple lines. ([@diogoosorio][])
 * [#248](https://github.com/rubocop-hq/rubocop-rails/pull/248): Fix a false positive for `Rails/SaveBang` when `update` is called on `ENV`. ([@eugeneius][])
+* [#251](https://github.com/rubocop-hq/rubocop-rails/pull/251): Fix a false positive for `Rails/FilePath` when the result of `Rails.root.join` is interpolated at the end of a string. ([@eugeneius][])
 
 ### Changes
 

lib/rubocop/cop/rails/file_path.rb

@@ -48,6 +48,7 @@ class FilePath < Cop
 
         def on_dstr(node)
           return unless rails_root_nodes?(node)
+          return unless node.children.last.str_type?
           return unless node.children.last.source.start_with?('.') ||
                         node.children.last.source.include?(File::SEPARATOR)
 

spec/rubocop/cop/rails/file_path_spec.rb

@@ -24,6 +24,14 @@
       end
     end
 
+    context 'when using Rails.root.join in string interpolation with nothing after it' do
+      it 'does not register an offense' do
+        expect_no_offenses(<<~'RUBY')
+          "#{Rails.root.join('log/production.log')}"
+        RUBY
+      end
+    end
+
     context 'when using string interpolation without Rails.root' do
       it 'does not register an offense' do
         expect_no_offenses(<<~'RUBY')
@@ -135,6 +143,14 @@
       end
     end
 
+    context 'when using Rails.root.join in string interpolation with nothing after it' do
+      it 'does not register an offense' do
+        expect_no_offenses(<<~'RUBY')
+          "#{Rails.root.join('log', 'production.log')}"
+        RUBY
+      end
+    end
+
     context 'when using string interpolation without Rails.root' do
       it 'does not registers an offense' do
         expect_no_offenses(<<~'RUBY')