StoreTest.test_reference_set_find is dependent on ordering

The results of test_reference_set_find are ordering dependent. The error lies here:

for bar in foo.bars.find():
        items.append((bar.id, bar.foo_id, bar.title))

items.sort()

...

self.assertTrue(objects[0] is bar)

Instead of referencing bar as set up in add_reference_set_bar_400, it is referencing whatever is the last item found in the for loop above, which may not be the intended object to compare to.