Вы говорите о select
или select_tag
или collection_select
?
# advanced users only
= f.select :field_id, options_for_select(Model.collect{|m| [m.name, m.id]})
# easiest, assuming you have a model
= f.collection_select :field_id, Model.all, :id, :name
# without model
= select_tag 'model[field_id]', @model.field_id, options_for_select(Model.collect{|m| [m.name, m.id]})