Alter Block render array and add another block's content

Question

I am attempting to attach a block to another block via hook_block_view_alter. The source block is a menu_block and the block to be attached is a view block, but the type of block shouldn't matter to the solution.

Here is the code used to attach the block. Assume that block_attached_block is populated with the bid of the block and at the point where I am altering the render array I have a fully rendered block.

function MODULE_block_view_alter(&$data, $block) {
  if (!empty($block->block_attached_block)) {

    // load the attached block from the database
    $attached_block = db_select('block', 'b')
      ->fields('b')
      ->condition('b.bid', $block->block_attached_block)
      ->execute()
      ->fetchObject();

    // render the block
    $block_content  = _block_render_blocks(array($attached_block));
    $build          = _block_get_renderable_array($block_content);
    $block_rendered = drupal_render($build);

    // attach the block
    $data['block_attached_block'] = array(
      '#type' => 'container',
      '#attributes' => array(
        'class' => array('block_attached_block'),
      ),
      'block' => array(
        '#markup' => $block_rendered,
      ),
    );
  }
}

If I render $data at this point I see the attached block rendered perfectly.

drupal_render($data);

However, when I load the page the attached block is missing. The documentation is pretty light.

My question is how do I get the rendered block to output on the page.

Answer 1

Here is an updated function taking into account some of the various types of blocks that we can attach blocks to.

function MODULE_block_view_alter(&$data, $block) {
  if (!empty($block->block_attached_block)) {

    // load the attached block from the database
    $attached_block = db_select('block', 'b')
      ->fields('b')
      ->condition('b.bid', $block->block_attached_block)
      ->execute()
      ->fetchObject();

    // render the block
    $block_content = _block_render_blocks(array($attached_block));
    $build         = _block_get_renderable_array($block_content);

    // create attached block render array
    $block_attached_block = array(
      '#type' => 'container',
      '#attributes' => array(
        'class' => array('block_attached_block'),
      ),
      'block' => $build,
    );

    // attach the block
    switch ($block->module) {

      // menu block module blocks
      case 'menu_block':
        $data['content']['#content']['block_attached_block'] = $block_attached_block;
        break;

      // views module blocks
      case 'views':
        $data['content']['#markup'] .= drupal_render($block_attached_block);
        break;

      // block module blocks
      // webform module blocks
      case 'block':
      case 'webform':
        $data['content'] .= drupal_render($block_attached_block);
        break;

      // all other module blocks
      default:
        $data['content']['block_attached_block'] = $block_attached_block;
        break;
    }
  }
}

Thanks Clive for getting me started on the right path.