Get old fieldtype data to prepend with new?

Question

I am trying to create a fieldtype that requires comparison to it's previous data. Any idea how to get this data in the save() function? I tried using $this->field_data but that just returns the "new" data submitted to the form.

To provide a tangible example, what I am trying to do is take the prior data and store it with the new data as an array so my save() function looks like this (if $this->field_data worked this should have):

function save($data) 
{ 
    if (!empty($this->field_data)) $data = json_decode($this->field_data, TRUE);
    $data[] = $data;


    return json_encode($data); 
}

Answer 1

This solution uses the channel_entries model:

function save($data) 
{
    if (ee()->input->post('entry_id'))
    {
        // look up old data
        ee()->load->model('channel_entries_model');
        $query = $this->EE->channel_entries_model
            ->get_entry(
                ee()->input->post('entry_id'), 
                ee()->input->post('channel_id')
            );
        $result = $query->row_array();

        // if exists set old data 
        if ($query->num_rows() > 0) $return = json_decode($result[$this->field_name], TRUE);

        // append new data
        $return[] = $data;

        return json_encode($return);
    }

    return "";
}

Answer 2

The only way to get the old data is to query for it:

//are we saving a new entry or updating an existing?
$entry_id = ee()->input->get('entry_id');

if ($entry_id)
{
  //get the old data from the database
  $query = ee()->db->select('field_id_'.$this->field_id)
                   ->where('entry_id', $entry_id)
                   ->get('channel_data');

  $old_data = $query->row('field_id_'.$this->field_id);

  $query->free_result();
}