Merge pull request rails#49136 from fatkodima/fix-counter-caches-var-modification

Fix in-place modification of shared `_counter_cache_columns` class attribute

Author: kamipo

activerecord/lib/active_record/associations/builder/belongs_to.rb

@@ -37,7 +37,7 @@ def self.add_counter_cache_callbacks(model, reflection)
       }
 
       klass = reflection.class_name.safe_constantize
-      klass._counter_cache_columns << cache_column if klass && klass.respond_to?(:_counter_cache_columns)
+      klass._counter_cache_columns |= [cache_column] if klass && klass.respond_to?(:_counter_cache_columns)
     end
 
     def self.touch_record(o, changes, foreign_key, name, touch) # :nodoc:

activerecord/test/cases/counter_cache_test.rb

@@ -3,6 +3,7 @@
 require "cases/helper"
 require "models/topic"
 require "models/bulb"
+require "models/person"
 require "models/car"
 require "models/aircraft"
 require "models/wheel"
@@ -12,7 +13,6 @@
 require "models/categorization"
 require "models/dog"
 require "models/dog_lover"
-require "models/person"
 require "models/friendship"
 require "models/subscriber"
 require "models/subscription"
@@ -397,6 +397,11 @@ class ::SpecialReply < ::Reply
     end
   end
 
+  test "counter_cache_column?" do
+    assert Person.counter_cache_column?("cars_count")
+    assert_not Car.counter_cache_column?("cars_count")
+  end
+
   private
     def assert_touching(record, *attributes)
       record.update_columns attributes.index_with(5.minutes.ago)