How to raise a warning message from the model

Hi there,

In my rails app I have the parent model ORDERS with SALES as a child (as nested routes). So 1 Order has_many Sales and 1 Sale belongs_to Order. In my USER model I have the column "maxdiscount". This is the place where a manager types in a value, like 100 $.

Sales agents can give a discount on a SALE. To check if the entered value from the sales agent is not higher than the threshold defined in the maxdiscount I did this:

def discount=(discount)
 if discount.present?
   if current_user.maxdiscount >= discount.to_d
     discount = discount.gsub(",", ".") #gsub for changing comma-seperated values into dot-seperated.
     self[:discount] = discount
   end
 end
end

The check works as intended, but when the if-clause is true like the sales agent typed in a value higher than his maxdiscount threshold then the discount does not get saved. Which is kind of ok.. What I want to do is to raise a warning with a flash message but this does not work. So far I understood that you can't raise error messages from the model.
This is what I did, but that is not working :/

 def discount=(discount)
  if discount.present?
    if current_user.maxdiscount >= discount.to_d
     discount = discount.gsub(",", ".")
     self[:discount] = discount
    else
      self.errors.add(:base, "The discount is too high. The product was added without it.")
     end
   end
  end

Any suggestions? Thanks in advance!