I originally thought this was returning a list instead of a single value in either direction. Since you only want one value, it would likely be simpler and faster to do two queries, with a 'limit 1' each time. Shipping data across the network between the DB and Django/Python is much slower than having the DB do more work. It'd also simplify the find/slice code (which appears to fetch every case (large data fetch), turn it into a list, search the list for the ID (linear time), then grab the one just before and after the index (each linear time search for linked list)). Let the DB can do the selecting/sorting work for you to ship and process less data (though my approach does add a second DB hit).

Something like:
```python
prev_test_id = TestCase.objects.filter(folder_id=folder_id, id__lt=testcase_id).order_by('-id').values_list("id", flat=True)[0]
next_test_id = TestCase.objects.filter(folder_id=folder_id, id__gt=testcase_id).order_by('id').values_list("id", flat=True)[0]
return (prev_test_id, next_test_id)
```
You'd have to handle when nothing was found (it'd throw an exception) and work in your TestplanTestcase link stuff).
The [0] converts it to a limit 1 SQL query, which only asks mysql to return a single row, and the order_by makes sure you get the one you want in each case.

Anyway - this is probably OK as-is for now, but if it seems to slow things down, it's a place to look and something fun to think about.