random_reference to just_once objects

[prescod]

[renatoliveira]

I believe I have the same requirement as described in the title of this issue.

In my case, I have a constraint on opportunity line items (yes, a Salesforce situation) in which only one instance of a product is allowed in the opportunity. So if I generate a list of random products, I want them to appear just once in all opportunities it ends up showing up randomly. That way I won't have two opportunity line items with the same product for the same opportunity.