In the part 1 of the Doctrine Tutorial we got a sweet setup with Doctrine: entities, queries, migrations and fixtures.
But, we left out one big piece: database relations. Once you've mastered these, you'll be unstoppable. Unfortunately, a lot of information out there make Doctrine relations look over-complicated. Actually, they're simple and beautiful. Sure, they have some pitfalls - but they'll also help you build a complex data structure in no time.
... and more - like some magic with the "Param Conversion".