mysqli and nested transactions

trans_start and trans_complete if for automatic commit or rollback based on the success or failure of your queries.

trans_begin, trans_rollback and trans_complete if for managing the transaction manually (you decide if and when you call rollback or commit).

You can't mix both and expect it to work. The doc is clear on that :

Note: Make sure to use $this->db->trans_begin() when running manual transactions, NOT $this->db->trans_start().

See this page for more details on transactions : http://www.codeigniter.com/user_guide/da...tions.html