nested attributes with validate parent

gistfile1.rb

#app/models/parent.rb
class Parent < ActiveRecord::Base
  accepts_nested_attributes_for :children
  validate :children_are_less_than_two

private
  def children_are_less_than_two
    errors.add(:base,
      "shouldn't have more than two children"
    ) if children.count > 2
  end
end

#app/models/child.rb
class Child < ActiveRecord::Base
  belongs_to :parent
end

#spec/models/parent_spec.rb
describe Parent do
  it "should accept nested attributes for children and validate parent before saving a child" do
    parent = Parent.new(children_attributes => {
      0 => {},
      1 => {},
    }).save.should be true

    Child.new(:parent => parent).save.should be false
  end
end