class Parent
  include Mongoid::Document

  field :name, :type => String

  embeds_many :children

  accepts_nested_attributes_for :children, :allow_destroy => true
end

class Child
  include Mongoid::Document

  field :position, :type => Integer
  field :name, :type => String

  validates_uniqueness_of :position

  embedded_in :parent
end

 :position

parent = Parent.create name: "Some Parent"

child_a = Child.new position: 0, name: "A"
child_b = Child.new position: 2, name: "B"
child_c = Child.new position: 3, name: "C"

parent.children = [child_a, child_b, child_c]
parent.save!

parent.update_attributes(
  "children_attributes" => {
    "0" => {
      "id" => child_a._id,
      "name" => "AA",
      "position" => 0
    },
    "1" => {
      "id" => child_b._id,
      "name" => "BB",
      "position" => 1,
      "_destroy" => "1"
    },
    "2" => {
      "id" => child_c._id,
      "name" => "CC",
      "position" => 2
    }
  }
)

puts parent.valid? # => false

 :position