foreign key select

hi all
how can i add a select of categories in my form if i have a relation
:photo belongs_to :category?
i don't know how can i access to category model from photo.rb
tnx

how can i add a select of categories in my form if i have a relation
:photo belongs_to :category?

http://api.rubyonrails.org/classes/ActionView/Helpers/FormOptionsHelper.
html#M000399

Roderick