Note that we dup the entire reflection instance

lib/active_model/serializer.rb

@@ -347,7 +347,7 @@ module ActiveModel
       return Enumerator.new {} unless object
 
       Enumerator.new do |y|
-        (@reflections ||= self.class._reflections.deep_dup).each do |key, reflection|
+        (self.instance_reflections ||= self.class._reflections.deep_dup).each do |key, reflection|
           next if reflection.excluded?(self)
           next unless include_directive.key?(key)
 
@@ -411,6 +411,6 @@ module ActiveModel
 
     protected
 
-    attr_accessor :instance_options, :reflections
+    attr_accessor :instance_options, :instance_reflections
   end
 end

lib/active_model/serializer/reflection.rb

@@ -151,6 +151,9 @@ module ActiveModel
       # @yield [ActiveModel::Serializer]
       # @return [:nil, associated resource or resource collection]
       def value(serializer, include_slice)
+        # NOTE(BF): This method isn't thread-safe because the _reflections class attribute is not thread-safe
+        # Therefore, when we build associations from reflections, we dup the entire reflection instance.
+        # Better solutions much appreciated!
         @object = serializer.object
         @scope = serializer.scope