Using Oracle without sequences

Hi all,

I have to set the primary key myself and not use sequences in an
Oracle schema

In http://thread.gmane.org/gmane.comp.lang.ruby.rails/118516 someone
writes,

"Correct, that's the expected/correct behavior -- so long as you
define
the PK properly, and set it yourself, it won't try to use a sequence."

I have

CREATE TABLE d_simulationproducts (
  id NUMBER(38,0) NOT NULL,
  simulationproducttype VARCHAR2(50) DEFAULT NULL NULL,
  position NUMBER(38,0) DEFAULT NULL NULL,
  productcategory VARCHAR2(50) DEFAULT NULL NULL,
  simulationproductname VARCHAR2(255) NULL,
)

ALTER TABLE d_simulationproducts
  ADD PRIMARY KEY (
    id
  )

SimulationProduct.columns

=> [#<ActiveRecord::ConnectionAdapters::OracleColumn:0x52bf750
@primary=true, @scale=0, @name="id", @null=false, etc...>

and do not override the id accessor in my model, nor manipulatig
set_sequence_name

When I try following in the console

SimulationProduct.create(:id=>1000)

it raises me the error : OCIError: ORA-02289: sequence does not exist

What am I doing wrong ?

Any hint is really appreciated,
Michael