Setting managed file default value from ajax request

Question

I'm trying to create a form using the Form API with the following behavior:

For a custom slider module, users can choose to upload an image which is saved as a managed file. This works fine. They also have the option to populate the slide content (slide title,link and image) from an existing node of a custom type. The node is selected from an auto-populated select field which on change calls an ajax function to auto-populate the title and subtitle fields. If the selected node has an image attached to it, a checkbox is displayed allowing the user to use the already uploaded image associated with node as the slider image as well. The problem I have is that I can't get the manged_file field to accept the nodes file id (fid) and re-render as managed_file with a default_value before returning it from the callback.

Here's my callback:

function load_image_callback($form,&$form_state) {
    // Load node from the selected value
    $node = node_load($form_state['values']['slide_nid'];

    // Set the managed_file default value to the nodes main image file
    $form_state['complete form']['slide']['slide_image']['slide_fid']['#default_value'] = $node->field_main_image[$node->language][0]['fid'];

    // Return the updated slide image wrapper to replace the existing one
    return $form_state['complete form']['slide']['slide_image'];
}

Answer 1

So after digging through Drupal core code I arrived at this solution:

In my ajax callback I load the actual file, set some managed file settings in the managed_file array, and then physically call the file_managed_file_process() function to setup the managed file display field array elements.

function load_image_callback($form,&$form_state) {
    // Load node from the selected value
    $node = node_load($form_state['values']['slide_nid'];
    $file = file_load($node->field_main_image[$node->language][0]['fid']);

    // Set the managed_file default value to the nodes main image file
    $form_state['complete form']['slide']['slide_image']['slide_fid']['#value'] = array('fid' => $file->fid);
    $form_state['complete form']['slide']['slide_image']['slide_fid']['#file'] = $file;

    // Process the managed file field with the set element
    $form_state['complete form']['slide']['slide_image']['slide_fid'] = file_managed_file_process($form_state['complete form']['slide']['slide_image']['slide_fid'], $form_state, $form);

    // Return the updated slide image wrapper to replace the existing one
    return $form_state['complete form']['slide']['slide_image'];
}