Failed open the sqlite3 db use sequel under padrino!

I use Padrino to develope a sample blog followed the tutorial in the
offical web.
First I use Sequel and sqlite3, all the rake task of migrates failed for
the message of ‘can not open the database’. I do’t know where I was
wrong and How to resolve. So I then use ActiveRecord and sqlite3 do the
tutorial and it works well.

Please some advices!