class Foo
  include Mongoid::Document

  field :pin,         type: String
  field :correct_pin, type: Boolean, default: false

  embedded_in :bar

  validate :valid_pin

  private

  def valid_pin
    return if correct_pin
    if validate_pin
      update_attributes(correct_pin: true, pin: nil)
    else
      errors.add(:pin, "Incorrect PIN")
    end
  end

  def validate_pin
    true
  end
end

class Bar
  include Mongoid::Document

  embeds_many :foos
end

b = Bar.create
Foo.create(pin: "1234", bar: b)

Moped::Errors::OperationFailure: The operation: #<Moped::Protocol::Command
  @length=90
  @request_id=7
  @response_to=0
  @op_code=2004
  @flags=[]
  @full_collection_name="example_development.$cmd"
  @skip=0
  @limit=-1
  @selector={:getlasterror=>1, :safe=>true}
  @fields=nil>
failed with error 10141: "Cannot apply $push/$pushAll modifier to non-array"

See https://github.com/mongodb/mongo/blob/master/docs/errors.md
for details about this error.

b.foos #=> [#<Foo _id: 5434497f8d46b9cc35000002, pin: nil, correct_pin: true>]
b.reload #=> #<Bar _id: 5434497f8d46b9cc35000001, >
b.foos #=> [#<Foo _id: 5434497f8d46b9cc35000002, pin: nil, correct_pin: true>]