Question

I am trying to allow the user to create a shortlist, however I add one thing the shortlist, and it overwrites the whatever else is in there, what am I doing wrong?

function validate_add_cart_item($id){

    $this->db->select('candidates.candidate_id, candidates.first_name, candidates.surname, candidates.DOB, candidates.talent, candidates.location, candidates.availability_order, candidates.availability, candidates.availability_comments, candidate_assets.url, candidate_assets.asset_size')
    ->from('candidates')
    ->join('candidate_assets', 'candidate_assets.candidates_candidate_id = candidates.candidate_id', 'left')
    ->where('candidate_assets.asset_size', 'small')
    ->where('candidate_assets.asset_type', 'image')
    ->where('candidates.candidate_id', (int)$id)
    ->limit(1)
    ->order_by('candidates.availability_order', 'DESC');

    $query = $this->db->get();

    //die($this->db->last_query());
    // Check if a row has been found
    if($query->num_rows > 0 ){
        foreach ($query->result() as $row)
        {
            $data = array(
                'id'      => $id,
                'name'    => $row->first_name." ".$row->surname,
                'location' => $row->location,
                'talent' => $row->talent,
                'image'  => $row->url
            );

            $this->session->set_userdata('shortlist', array($data));
            return TRUE;
        }

    // Nothing found! Return FALSE! 
    } else {
        return FALSE;
    }

Answer 1

   foreach(...){
      // ...
     $this->session->set_userdata('shortlist', array($data));
   }

You are overwriting your previous userdata several times. Instead of this you should create an array, and call set_userdata once.

  $udata = array();
  foreach(...){
    // ...
    $udata []= $data;
  }
  $this->session->set_userdata('shortlist', $udata);

edit:

And your query returns only one row, you probably want to update your array, not overwrite it, so something like this:

  $udata = $this->session->get_userdata('shortlist');
  foreach(...){
    // ...
    $udata []= $data;
  }
  $this->session->set_userdata('shortlist', $udata);