Support one-to-many relationships for exportable tables

The new ability to export objects from joined tables is awesome! But it would be even more awesome if we could support exporting objects which joins a one-to-many relationship table.

I found that it's not that hard to do this. The only API addition we need is that each join that is a one-to-many relationship needs to define array_key like this:

  'join' => array(
    'term' => array(
      'table' => 'ctools_plugin_example_object_term',
      'left_key' => 'oid',
      'right_key' => 'oid',
      // One-to-many relationships must define 'array_key'. Can be either
      // EXPORT_NUMERIC_ARRAY or a field name from the joined table.
      'array_key' => EXPORT_NUMERIC_ARRAY,
      'load' => array(
        'name',
        'synonym',
      ),
    ),
  ),

The export should then look something like this:

  $object = new stdClass;
  $object->disabled = FALSE; /* Edit this to true to make a default object disabled initially */
  $object->api_version = 1;
  $object->machine_name = 'example';
  $object->title = 'Example';
  $object->body = 'Hello world';
    $term = new stdClass;
    $term->name = 'Lorem';
    $term->synonym = '';
  $object->term[] = $term;
    $term = new stdClass;
    $term->name = 'Ipsum';
    $term->synonym = '';
  $object->term[] = $term;

The join key is used as the identifier for child objects ($term).

Attached is two patches. One with only the API addition and one that adds an example module implementing this and an export_ui plugin. The example module is working, but not at all complete. Just so you can take a look at it.