Validates presence of each other in two associated models - ruby-on-rails

Question

I have the following two models:
class Parent < ActiveRecord::Base
has_one :child, dependent: :destroy
validates :child, presence: true
end
class Child < ActiveRecord::Base
belongs_to :parent
validates :parent, presence: true
end
I want to create Parent object.
If I do the following:
Parent.create! or Factory(:parent)
Exception raises: ActiveRecord::RecordInvalid: Validation failed: Child can't be blank
But I can't create Child object without Parent object for the same reason - I need to create Parent object first in order to pass presence validation.
As it appears I have some kind of infinite recursion here.
How to solve it?

Answer 1

UPDATE:
The code below works well in my environment ( Rails3.2.2, ruby 1.8.7)
# parent.rb
class Parent < ActiveRecord::Base
has_one :child
validates :child, :presence => true
end
# child.rb
class Child < ActiveRecord::Base
belongs_to :parent
validate :parent, :presence => true
end
# parent_test.rb
require 'test_helper'
class ParentTest < ActiveSupport::TestCase
test "should be saved" do
parent = Parent.new(:name => "111")
child = Child.new(:name => "222", :parent => parent)
parent.child = child
parent.save!
puts "after saved, parent: #{parent.inspect}"
puts "after saved, child: #{child.inspect}"
assert parent.id > 0
assert child.id > 0
end
end
run this test and got:
Started
after saved, parent: #<Parent id: 980190963, name: "111", created_at: "2012-04-05 23:19:31", updated_at: "2012-04-05 23:19:31">
after saved, child: #<Child id: 980190963, name: "222", parent_id: 980190963, created_at: "2012-04-05 23:19:31", updated_at: "2012-04-05 23:19:31">
.
Finished in 0.172716 seconds.
1 tests, 2 assertions, 0 failures, 0 errors
PREVIOUS ANSWER ================
try to initialize them separately, then add the association, at last save them.
parent = FactoryGirl.build(:parent)
child = FactoryGirl.build(:child, :parent => parent)
parent.child = child
parent.save
child.save # seems this line of code is redundant? I am not sure.
for more details of "build, create", see its official website: https://github.com/thoughtbot/factory_girl/blob/master/GETTING_STARTED.md