Suppress child record name in Rails validation message when using fields_for
===

I have two models, Pricing and Pricing Details.  Pricing has_many pricing_details.  I have a form that uses form_for so I can create the pricing object and details all in the same form, and save all in one save call.

All the code functions as expected, the problem is, the validation message when I leave out a validated field on the pricing_details model looks like this:

"Pricing details cost Annual Price cannot be blank"

My validation is:

    def validate_cost
      field_name = PriceType.find(self.price_type_id).name
      if self.cost.blank?
        self.errors.add :cost, field_name + " cannot be blank"
      end
    end

What I want is simply:
"Annual Price cannot be blank"

Is there some hack to the locale that I can use to suppress the child model and field name?  I've tried adding to base instead, like this:

    self.errors.add :base, field_name + " cannot be blank"

But then I just get "Pricing details base Annual Price cannot be blank".