Fix UnusedIgnoredColumns detection when using +=

Author: lucthev

changelog/fix_unused_ignored_columns_plus_equals.md

@@ -0,0 +1 @@
+* [#895](https://github.com/rubocop/rubocop-rails/pull/895): Fix `Rails/UnusedIgnoredColumns` not recognizing columns added via `+=`. ([@lucthev][])

lib/rubocop/cop/rails/unused_ignored_columns.rb

@@ -28,12 +28,16 @@ class UnusedIgnoredColumns < Base
           (send self :ignored_columns= $array)
         PATTERN
 
+        def_node_matcher :appended_ignored_columns, <<~PATTERN
+          (op-asgn (send self :ignored_columns) :+ $array)
+        PATTERN
+
         def_node_matcher :column_name, <<~PATTERN
           ({str sym} $_)
         PATTERN
 
         def on_send(node)
-          return unless (columns = ignored_columns(node))
+          return unless (columns = ignored_columns(node) || appended_ignored_columns(node))
           return unless schema
 
           table = table(node)
@@ -43,6 +47,7 @@ def on_send(node)
             check_column_existence(column_node, table)
           end
         end
+        alias on_op_asgn on_send
 
         private
 

spec/rubocop/cop/rails/unused_ignored_columns_spec.rb

@@ -105,6 +105,17 @@ class User < ApplicationRecord
         RUBY
       end
     end
+
+    context 'when using addition assignment on ignored_columns' do
+      it 'registers an offense to the nonexistent column' do
+        expect_offense(<<~RUBY)
+          class User < ApplicationRecord
+            self.ignored_columns += ['real_name']
+                                     ^^^^^^^^^^^ Remove `real_name` from `ignored_columns` because the column does not exist.
+          end
+        RUBY
+      end
+    end
   end
 
   context 'with no tables db/schema.rb' do