I think you' better write a method rather than using belongs_to.
In my opnion, if you use belongs_to :xxx, than you get xxx for all the
object of Category class, not just the root category.
If you write a seperate method, you can write your own logic in it,
for example, for every category, you first find its root category, and
then return the section object of that root category.
And I think you can just use that as the same as by using belongs_to