I am starting a new (and first) project with Symfony2, but I didn't choise Database engine yet. And it could be MySQL or NoSQL engines like MongoDB or others, or also it could be changed along the proyect cycle. So, I would like to start working without limitation of choising Doctrine, Doctrine for Mongo or others. And allowing posibility of changing only model layer without big impact on controllers, and classes.
Recomendations? Comments? Best Practices?