If Rails typically hides SQL details from you, requiring you to walk the tree, this becomes a problem.
ActiveRecord (the Rails ORM) is just a very thin layer of code on top of SQL. This makes it dirt easy to customize the SQL for fetching or updating. I don't think this will be a problem.
However, I'm not a heavy-duty user of AR, so it will be fun to play with this and see exactly how this all falls out.