How to get average on Mongoid when using Money-Rails

Question

I'm using Money-Rails as field like so:

class Product
  ...
  field :price_GBP, type: Money
  ...
end

And I'm trying to get the average for all price_GBP documents. If the price_GBP field were a float or integer I would something like this:

avg = Product.avg(:price_GBP)

And that would provide me the average price for all products. I've tried:

avg = Product.avg(":price_GBP.cents") #or
avg = Product.avg("price_GBP.fractional")

But neither works. Any ideas?

EDIT:

Here's the price_GBP attribute:

price_GBP: {
    cents: 220,
    currency_iso: "GBP"
},