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  endend

It's not a bug. Acording to the documentation

Validates that the specified attributes are not blank (as defined by Object#blank?)

and validates :children, :presence => true is just the the same. The documentation doesn't say what happens if you try to use it on an association. You should use custom validation using validate.

Using validates_presence_of on has_many association calls blank? on association children, which is an object of class Array. Since the blank? is not defined for an Array, it fires method_missing which is caught inside Rails. Usually it do what you wants but I found it fails in Rails 3.1rc and Ruby 1.8.7 in a really awful way: it silently reverts the changes of associated records. It took me a couple of hours to find out what's happening.