testing question

ok, i've written a couple of unit tests and it was pretty straight
forward as far as what i should and shouldn't be testing in the unit
test.. until now.

i am testing a customer model right now.. i figure that a unit test
should test the functions of a model as well as if you can do all the
crud stuff and validations. right now i wish to test a customer model
function that returns what level of customer they are.. this is based
on what things they've ordered in the past. so my question: is it ok to
bring in other models when doing unit tests? i figure i will have to
load at least 3 other test fixtures to test this out (orders,
order_items, and inventory), but not even sure if i can actually use
the other models in the customer unit test because i tried putting test
= Order.new in my test and it failed on that line.. i'm starting to
think that maybe this aspect shouldn't be tested in the unit test?

thanks in advance

stuart

I often pull in other models (via fixtures). Fixtures are all "fake"
made up data anway, and it's a rare app where every model exists in
isolation...