Tree with a function field

def _getmytest(self, cr, uid, ids, field_name, arg, context):
        res = {}
        #dumy function that returns only the ids
        for a in ids:
            res[a] = a
        return res

 _columns = {

        'mytest': fields.function(_getmytest, type='char', size=512, method=True, string='thats my test'),
    }