using transactions

You probably need to run the transactions manually in this context. In other words, you start with trans_begin() instead of trans_start(), then use trans_rollback() and trans_commit() to control whether the transaction is rolled back or committed. You also don't use trans_complete() for manual transactions.

When running the transactions manually, remember to check trans_status() before calling trans_commit(). Essentially, this is what trans_complete() normally does (chooses whether to call trans_rollback() or trans_commit() based on the result of trans_status()), but controlling the transactions manually allows you to rollback or commit the transaction based on your own logic.

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide