In Mongoid::Validations::AssociatedValidator on line 28 there should be merge instead of merge!. Since the options hash is frozen merge! won't work. Action Record (which is very similar) is using merge.

Below is a modified copy with the correct code. Don't have git so I can't create a pull request.

1. encoding: utf-8
   module Mongoid #:nodoc:
   module Validations #:nodoc:
2. Validates whether or not an association is valid or not. Will correctly
3. handle has one and has many associations.
   #
4. Example:
   #
5. class Person
6. include Mongoid::Document
7. embeds_one :name
8. embeds_many :addresses
   #
9. validates_associated :name, :addresses
10. end
    class AssociatedValidator < ActiveModel::EachValidator

1. Validates that the associations provided are either all nil or all
2. valid. If neither is true then the appropriate errors will be added to
3. the parent document.
   #
4. Example:
   #
5. <tt>validator.validate_each(document, :name, name)</tt>
   def validate_each(document, attribute, value)
   values = value.is_a?(Array) ? value : [ value ]
   return if values.collect { |doc| doc.nil? || doc.valid? }

   .all?
   document.errors.add(attribute, :invalid, options.merge(:value => value))
   end
   end
   end
   end