just wrap as a sql literal

activerecord/lib/active_record/relation/predicate_builder.rb

@@ -26,8 +26,7 @@ def self.build_from_hash(engine, attributes, default_table)
           when Range, Arel::Relation
             attribute.in(value)
           when ActiveRecord::Base
-            sanitized_id = attribute.class == Arel::Attributes::String ? value.id : value.quoted_id
-            attribute.eq(sanitized_id)
+            attribute.eq(Arel.sql(value.quoted_id))
           when Class
             # FIXME: I think we need to deprecate this behavior
             attribute.eq(value.name)