class Price

def initialize(value = 0, currency = '')
  @value,@currency = value, currency
end
def mongoize
  [@value,@currency.to_s]
end

class << self
  def demongoize(object)
    if object
      cur = object[1] || ''
      Price.new(object[0], cur)
    else
      Price.new()
    end
  end
  def mongoize(object)
    case object
    when Price then object.mongoize
    when Array then Price.new(object[0], object[1]).mongoize
    else object
    end
  end

  def evolve(object)
    case object
    when Price then object.mongoize
    else object
    end
  end
end

class Financial
  include Mongoid::Document

  field: quote, type: Price
  field: tree, type: Hash
end

revenue = Price.new(10,'USD')

 record = Financial.new()
 record.quote = revenue
 record.save # All works so far

 record.tree = { data: [10,'USD']}
 record.save # this works too

 record.tree = { data: revenue}
 record.save # here I get the undefined bson_type for Price error.