Relationship data getting cleared when using Channel Entry API

When I use:

ee()->api_channel_entries->save_entry($data, $channel_id, $entry_id);

with data such as this:

$data = array( ‘title’ => ‘Breaking News Story!’, ‘entry_date’ => ‘1256953732’, ‘edit_date’ => ‘1351653729’, ‘field_id_6’ => ‘Some data’, ‘field_ft_6’ => ‘none’, ‘field_id_19’ => ‘More data’, ‘field_ft_19’ => ‘xhtml’ );

Any relationships that were present in the entry that was updated gets deleted. How can one prevent this?