silverstripe-blog/code/admin/GridFieldMergeAction.php

<?php

class GridFieldMergeAction implements GridField_ColumnProvider, GridField_ActionProvider {
	/**
	 * List of records to show in the MergeAction column.
	 *
	 * @var array|SS_List
	 */
	protected $records;

	/**
	 * Type of parent DataObject (i.e BlogTag, BlogCategory).
	 *
	 * @var string
	 */
	protected $parentType;

	/**
	 * Relationship method to reference parent (i.e BlogTags).
	 *
	 * @var string
	 */
	protected $parentMethod;

	/**
	 * Relationship method to reference child (i.e BlogPosts).
	 *
	 * @var string
	 */
	protected $childMethod;

	/**
	 * @param array|SS_List $records
	 * @param string $parentType
	 * @param string $parentMethod
	 * @param string $childMethod
	 */
	public function __construct($records = array(), $parentType, $parentMethod, $childMethod) {
		$this->records = $records;
		$this->parentType = $parentType;
		$this->parentMethod = $parentMethod;
		$this->childMethod = $childMethod;
	}

	/**
	 * {@inheritdoc}
	 */
	public function augmentColumns($gridField, &$columns) {
		if(!in_array('MergeAction', $columns)) {
			$columns[] = 'MergeAction';
		}

		return $columns;
	}

	/**
	 * {@inheritdoc}
	 */
	public function getColumnsHandled($gridField) {
		return array('MergeAction');
	}

	/**
	 * {@inheritdoc}
	 */
	public function getColumnContent($gridField, $record, $columnName) {
		if($columnName === 'MergeAction' && $record->{$this->childMethod}()->Count() > 0) {
			$dropdown = new DropdownField('Target', 'Target', $this->records->exclude('ID', $record->ID)->map());
			
			$prefix = strtolower($this->parentMethod . '-' . $this->childMethod);

			$action = GridFieldFormAction::create(
				$gridField,
				'MergeAction' . $record->ID,
				'Move',
				'merge',
				array(
					'record' => $record->ID,
					'target' => $prefix . '-target-record-' . $record->ID,
				)
			);

			$action->setExtraAttributes(array(
				'data-target' => $prefix . '-target-record-' . $record->ID
			));

			return $dropdown->Field() . $action->Field() . '<a title="Move posts to" class="MergeActionReveal">move posts to</a>';
		}

		return null;
	}

	/**
	 * {@inheritdoc}
	 */
	public function getColumnAttributes($gridField, $record, $columnName) {
		return array('class' => 'MergeAction');
	}

	/**
	 * {@inheritdoc}
	 */
	public function getColumnMetadata($gridField, $columnName) {
		return array('title' => 'Move posts to');
	}

	/**
	 * {@inheritdoc}
	 */
	public function getActions($gridField) {
		return array('merge');
	}

	/**
	 * {@inheritdoc}
	 */
	public function handleAction(GridField $gridField, $actionName, $arguments, $data) {
		if($actionName === 'merge') {
			$controller = Controller::curr();

			$request = $controller->getRequest();

			$target = $request->requestVar($arguments["target"]);

			$parentType = $this->parentType;

			$fromParent = $parentType::get()->byId($arguments['record']);
			$toParent = $parentType::get()->byId($target);

			$posts = $fromParent->{$this->childMethod}();

			foreach ($posts as $post) {
				$relationship = $post->{$this->parentMethod}();

				$relationship->remove($fromParent);
				$relationship->add($toParent);
			}
		}
	}
}