How to create "custom" fields in Model?

How to create "custom" fields in Model? like sum, based on quantity
and price.
I guess it should be something like:

  def summ
    price * quantity
  end
Am I right?

If so, How to use them in :conditions parameter of the
ActiveRecord::Base::find method? Is it possible it all?

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V/r
Anthony Eden

ok, sorry