use the schema cache when asking for the primary key

activerecord/lib/active_record/attribute_methods/primary_key.rb

@@ -72,7 +72,7 @@ def get_primary_key(base_name) #:nodoc:
           when :table_name_with_underscore
             base_name.foreign_key
           else
-            if ActiveRecord::Base != self && connection.table_exists?(table_name)
+            if ActiveRecord::Base != self && connection.schema_cache.table_exists?(table_name)
               connection.primary_key(table_name)
             else
               'id'