ActiveRecord and legacy oracle tables

I'm trying to connect to an existing oracle database. Some tables in this database do not have a standard "id" field, or any other primary keys. As I understand it, I can specify which table a model refers to:

class MyModel < ActiveRecord::Base   set_table_name "t_mymodel" end

However using this method, I cannot perform a save command on my model. Attemnpting it yields this error:

ORA-02289: sequence does not exist

I thought by setting the sequence name to an empty string would solve it:

class MyModel < ActiveRecord::Base   set_table_name "t_mymodel"   set_sequence_name "" end

But that yields:

ORA-00936: missing expression

Any assistance would be most appreciative