When I go to Italy I always get recommendations from Biba Caggiano's Trattoria Cooking book. For Bologna she recommends:

Alla Grada Via della Grada, 6 Bologna

Birreria Lamma Via de' Giudei, 4 Bologna

Caminetto d'Oro Via dei Falegnami, 4 Bologna

Trattoria Leonida Vicolo Alemagna, 2 Bologna

Trattoria Boni Via Saragozza, 88.A Bologna

Trattoria Gianni Via Clavature, 18 Bologna

Da Nello Via Montegrappa,2 Bologna

Trattoria Lo Sterlino Via Murri, 71 Bologna

Her book was written in 1992 so it's a good idea to make sure that the restaurant that you want to go to is still in business--perhaps the front desk can look over the list and point out one that is nearby. As these are all trattorias they will all be reasonable in price and fairly casual (but do make reservations). I've eaten at a number of trattorias that are recommended in her book for Florence (Firenze) and Rome and they were all outstanding. Unfortunately, I haven't eaten at any in Bologna so I can't give you a first hand recommendation but I'd be very surprised if the restaurant isn't great. Happy eating!

Mark