Nested models and parent validation

This will probably work for you, but I have a feeling there's a much better answer out there. It sounds like a bug to me.

class Parent < ActiveRecord::Base
  validate :must_have_children

  def must_have_children
    if children.empty? || children.all?(&:marked_for_destruction?)
      errors.add(:base, 'Must have at least one child')
    end
  end
end