Can't use merb sessions with datamapper & mysql, get back an error about needing an id on the text column or something (I had the error a couple of days ago.)
I suggest grabbing merb_datamapper svn source to fix this in. To do so make sure you have subversion installed on your machine (I'm assuming a Unix based machine here.)
Checkout the source
svn co http://svn.devjavu.com/merb/plugins/merb_datamapper
Open up the affected file in your favourite editor (I use TextMate)
cd merb_datamapper mate lib/merb/sessions/data_mapper_session.rb
Find line 25 that contains
`property :session_id, :text, :lazy => false, :key => true`
:text, :lazy => falseto replace it with
`property :session_id, :string, :key => true`
Save and close the file, thats the editing done. Now to install the gem.
Build the gem
Install the gem
sudo gem install pkg/merb_datamapper-0.5.gem
And you're away with the fix installed. Now just run
merb to create your sessions table in the db. Hope this helped!