class GrantApplication < ActiveRecord::Base
  has_one :household, :dependent => :destroy
end

class Household < ActiveRecord::Base
  belongs_to :grant_application
end

map.resources :grant_applications do |grant|
  grant.resource :household
end

<% form_for([:grant_application, @household]) do |f| %>

undefined method 'grant_households_path' for #<ActionView::Base:0x23eda44>

<% form_for(@household, :url => grant_application_household_path(@grant) ) do |f| %>