Storing arrays in db + model architecture


My users can select items and define a quantity attribute for those.
Those items can be selected with checkboxes. And the quantity is a field

Collecting all in one form; the checkbox selections, and other details,
the info should be stored in the db. Resulting in a purchase order for
example. A single user wouldn’t have many purchase orders in the system
at one time.

My question is how to store the resulting array in the db? I know how to
get the array of values but what should I use as the container in the
db, a single row with a sting hash, store each item separately,
something else? How can I relate the quantity for each item?

The available items are not predefined, and users can create new items
into the system also.

I think it is pretty trivial stuff but I would like to get the models
and logic right from the beginning. Any good suggestions/tutorials?


Oppps! This should have gone to the Rails area.
I will move it there.