Create string primairy key

Hi all,

I have 2 questions:
I try to make a primairy key that's a string (tuis not the default
int). I use:
t.column "id", :string, {:limit => 30, :null =>
false, :auto_increment => false}

When migrating this gives an error (very generic one). Can someone
lease explain the problem with that line?

When I have a many-to-many (users-groups) relationship I have a
coupling table groupsusers that contain the id of the group and user.
is it normal that you give these 2 colums an unique key?

Many thanks