如何在codeigniter中将数据库错误从模型传递到控制器

Question

I have something like voting database, so I made my database like this Vote_table:

user_id
product_id
vote
primary key is both user_id & product_id

so that no one can vote for a product twice, and this is my model:

public function add_vote_records($product_id, $user_id)
{
    $this->db->set('product_id', $product_id);
    $this->db->set('members_id', $user_id);
    $query = $this->db->insert('product_votes');
    if ($query->num_rows() == 0)
    return FALSE;

    return $query->result_array();
}

and this is my controller:

if ($this->products_model->add_vote_records($product_id, $user_id)) {
    $this->product($product_id);
} else {
    $this->session->set_flashdata('flash_message', 'Sorry you already voted.');
    $this->product($product_id);
}

but it gives me this error when im trying to vote when I already voted:

Duplicate entry '8-2' for key 'PRIMARY'

And that's what I really want, but I want to give the user another message my own message instead of this error.