primary key and partition

[jason zhang]

below is my table schema:

create table test (id int(11) primary key, region_id int(11));

I'd like to partion the table with hash(region_id) because my data
is organize by regions. But mysql force to include the primary key
when partition the table.

How should I handle this? create a primary key include both id and region_id?
Does that make sense? The region_id is not unique.