bar.rb

class Bar < ActiveRecord::Base
  attr_accessible :name
  has_many :bazs, autosave: true
end

baz.rb

class Baz < ActiveRecord::Base
  belongs_to :bar
  attr_accessible :name
end

gistfile1.sh

$ rails c

irb(main):001:0> bar = Bar.first
  Bar Load (0.4ms)  SELECT "bars".* FROM "bars" LIMIT 1
=> #<Bar id: 1, name: "FooBar", created_at: "2013-09-19 09:28:55", updated_at: "2013-09-19 09:28:55">

irb(main):002:0> baz = bar.bazs.first
  Baz Load (0.5ms)  SELECT "bazs".* FROM "bazs" WHERE "bazs"."bar_id" = 1 LIMIT 1
=> #<Baz id: 1, name: "Baz 1", bar_id: 1, created_at: "2013-09-19 09:28:55", updated_at: "2013-09-19 09:37:42">

irb(main):003:0> baz.name = 'New Baz'
=> "New Baz"

irb(main):004:0> bar.save!
   (0.2ms)  BEGIN
   (0.1ms)  COMMIT
=> true

seeds.rb

Bar.create do |bar|
  bar.name = 'FooBar'
  bar.bazs.build do |baz|
    baz.name = 'Baz 1'
  end
end