how to get last insert id

Try to ans, hopefully this solution can solve u're problem:

In CI-3, i do like this:
Field 'Id' => Primary Key and Auto Increment

Code:

//MODEL

function get_table(){
        $table = "tbl_name";
        return $table;
    }

function _insert($data){
        $table = $this->get_table();
        $this->db->insert($table, $data);
    }

function get_max(){
        $table = $this->get_table();
        $this->db->select_max('Id');
        $query = $this->db->get($table);
        return $query;
    }

//CONTROLLER

function bla(){
...
//INSERT NEW DATA:
$this->model_name->_insert($data);
$update_id = $this->model_name->get_max();
$this->resp_json['id'] = $update_id
...

}