we know the table and pk, so we can calculate a default sequence name

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -447,11 +447,12 @@ def insert_sql(sql, name = nil, pk = nil, id_value = nil, sequence_name = nil)
 
         super
 
+        return unless pk
+
         # If a pk is given, fallback to default sequence name.
         # Don't fetch last insert id for a table without a pk.
-        if pk && sequence_name ||= default_sequence_name(table, pk)
-          last_insert_id(sequence_name)
-        end
+        sequence_name ||= "#{table}_#{pk}_seq"
+        last_insert_id(sequence_name)
       end
       alias :create :insert