Fix an error for Rails/RenderPlainText when the content type is passed as a constant

changelog/fix_error_rails_render_plain_text_constant.md

@@ -0,0 +1 @@
+* [#1325](https://github.com/rubocop/rubocop-rails/pull/1325): Fix an error for `Rails/RenderPlainText` when the content type is passed as a constant. ([@earlopain][])

lib/rubocop/cop/rails/render_plain_text.rb

@@ -53,9 +53,12 @@ def find_content_type(node)
           node.pairs.find { |p| p.key.value.to_sym == :content_type }
         end
 
-        def compatible_content_type?(node)
-          (node && node.value.value == 'text/plain') ||
-            (!node && !cop_config['ContentTypeCompatibility'])
+        def compatible_content_type?(pair_node)
+          if pair_node.nil?
+            !cop_config['ContentTypeCompatibility']
+          elsif pair_node.value.respond_to?(:value)
+            pair_node.value.value == 'text/plain'
+          end
         end
 
         def replacement(rest_options, option_value)

spec/rubocop/cop/rails/render_plain_text_spec.rb

@@ -19,6 +19,12 @@
       RUBY
     end
 
+    it 'does not register an offense when `content_type` is a constant' do
+      expect_no_offenses(<<~RUBY)
+        render text: 'Ruby!', content_type: Foo
+      RUBY
+    end
+
     it 'does not register an offense when using `render plain:`' do
       expect_no_offenses(<<~RUBY)
         render plain: 'Ruby!'