gistfile1.rb

# The models
class User < ActiveRecord::Base
  has_many :things
  accepts_nested_attributes_for :things
end

class Thing < ActiveRecord::Base
  belongs_to :user
  validates_presence_of :user_id
  validates_presence_of :name # For clarity below
end

# Trying to create a new user with a new thing
u = User.new(:things_attributes => [{:name => 'thing'}])
u.save #=> false
u.errors.full_messages #=> ["Things user can't be blank"]