I ran into a similar problem. I created a new ROR app that had to work with an existing legacy database (Oracle). The db has a members table. The primary key was already named 'mem_id'.
So, in my model I have: set_table_name "members" set_primary_key "mem_id"
set_sequence_name "mem_id_seq"
It works after I added set_sequence_name. I got the same Oracle 'sequence does not exist error' before I added that.
good luck. -emil