I think the problem is in how you are defining the related company schema:
JawboneUP3 = item(itemID = "1", name = "Jawbone UP3", description = "The latest UP!",
category = "tracker", price = "$174.99", company = "Jawbone")
# HERE^
The item
constructor expects a company
instance but you are passing a string value. Fix it:
JawboneUP3 = item(itemID="1",
name="Jawbone UP3",
description="The latest UP!",
category="tracker",
price="$174.99",
company=company(name="Jawbone"))