Create string primairy key

Hi all,

I have 2 questions: 1- 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?

2- 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 Stijn