Document query cache invalidation for raw_connection

Author: fxn

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -798,6 +798,10 @@ def clean! # :nodoc:
       #
       # This is useful for when you need to call a proprietary method such as
       # PostgreSQL's lo_* methods.
+      #
+      # Active Record cannot track if the database is getting modified using
+      # this client. If that is the case, generally you'll want to invalidate
+      # the query cache using `ActiveRecord::Base.clear_query_cache`.
       def raw_connection
         with_raw_connection do |conn|
           disable_lazy_transactions!