add validation error from associated model to parent

so basically i has this setup (example):

class FellowUser < ActiveRecord::Base
  belongs_to :fellow
  belongs_to :user
end

class Fellows < ActiveRecord::Base
  belongs_to :user
  has_many :fellow_users
  has_many :fellows, :through => :fellow_users
end

what i want, in rspec terms, is this: