DataMapper ORM v1.8.2

[eluser]WanWizard[/eluser]
You're confusing a business process (the ordering of products) with a database relation. You can relate two objects of a given model only once, like any two people can only married to each other once at the same time.

If you want to record how many items of this product where in this order, record the quantity in the joining table, as it is a property of the relation. Look up "join_fields" in the docs.