associations.rb

class Foo
  include Mongoid::Document
  include Mongoid::Timestamps
  has_many :bars
  has_many :bazs

  before_validation do
    puts "FOO VALIDATION"
  end
end

class Bar
  include Mongoid::Document
  include Mongoid::Timestamps
  belongs_to :foo

  before_validation do
    puts "BAR VALIDATION"
  end
end

class Baz
  include Mongoid::Document
  include Mongoid::Timestamps
  belongs_to :foo

  before_validation do
    puts "BAZ VALIDATION"
  end
end

$> rails c

> foo = Foo.create

> bar = Bar.new(:foo => foo)
> bar.save

> baz = Baz.new(:foo => foo)
> baz.save

> foo.save
FOO VALIDATION
BAR VALIDATION
BAZ VALIDATION
=> true

> foo.reload && foo.save
FOO VALIDATION
=> true