Has_many validation

Hi all,
I’ve got a pretty standard has_many relationship here. Customer has_many
items, and the association is being performed like so:

@customer.items << @item

My question regards the best way to perform validation when adding an
item to a customer. Specifically, I need to make sure that a certain
property of the @item matches that of the @customer. Does this type of
validation take place in the Customer model? If so, how? Or should I
create some wrapper around << in the Customer model, which would perform
this validation and created the association? What’s the preferred
method?

Thanks in advanced,
Doug