Magento 2 adminhtml multiselect and showing selected options after save - multi-select

Question

How do I get the Magento2 Adminhtml Form with a Multiselect to select the saved options?
Here is my Adminhtml Form Class for reference.
namespace RussellAlbin\Blog\Block\Adminhtml\Post\Edit;
/**
* Adminhtml blog post edit form
*/
class Form extends \Magento\Backend\Block\Widget\Form\Generic
{
/**
* #var \RussellAlbin\Blog\Model\Category\Source\ListCategories
*/
protected $_categories;
/**
* #var \RussellAlbin\Blog\Model\Postcategory
*/
protected $_postcategory;
/**
* #var \Magento\Store\Model\System\Store
*/
protected $_systemStore;
/**
* #param \Magento\Backend\Block\Template\Context $context
* #param \Magento\Framework\Registry $registry
* #param \Magento\Framework\Data\FormFactory $formFactory
* #param \Magento\Store\Model\System\Store $systemStore
* #param \RussellAlbin\Blog\Model\Category\Source\ListCategories $categories
* #param \RussellAlbin\Blog\Model\Postcategory $postcategory
* #param array $data
*/
public function __construct(
\Magento\Backend\Block\Template\Context $context,
\Magento\Framework\Registry $registry,
\Magento\Framework\Data\FormFactory $formFactory,
\Magento\Store\Model\System\Store $systemStore,
\RussellAlbin\Blog\Model\Category\Source\ListCategories $categories,
\RussellAlbin\Blog\Model\Postcategory $postcategory,
array $data = []
) {
$this->_categories = $categories;
$this->_systemStore = $systemStore;
$this->_postcategory = $postcategory;
parent::__construct($context, $registry, $formFactory, $data);
}
/**
* Init form
*
* #return void
*/
protected function _construct()
{
parent::_construct();
$this->setId('blog_post_form');
$this->setTitle(__('Blog Post Information'));
}
/**
* Prepare form
*
* #return $this
*/
protected function _prepareForm()
{
/** #var \RussellAlbin\Blog\Model\Post $model */
$model = $this->_coreRegistry->registry('blog_post');
/** #var \Magento\Framework\Data\Form $form */
$form = $this->_formFactory->create(
['data' => ['id' => 'edit_form', 'action' => $this->getData('action'), 'method' => 'post']]
);
$form->setHtmlIdPrefix('post_');
$fieldset = $form->addFieldset(
'base_fieldset',
['legend' => __('General Information'), 'class' => 'fieldset-wide']
);
if ($model->getPostId()) {
$fieldset->addField('post_id', 'hidden', ['name' => 'post_id']);
}
// Gather our existing categories
$currentCategories = $this->_getExistingCategories( $model );
// Get all the categories that in the database
$allCategories = $this->_categories->toOptionArray();
$field = $fieldset->addField(
'blog_categories',
'multiselect',
[
'label' => __('Categories'),
'required' => true,
'name' => 'blog_categories',
'values' => $allCategories,
'value' => $currentCategories
]
);
$form->setValues($model->getData());
$form->setUseContainer(true);
$this->setForm($form);
return parent::_prepareForm();
}
/**
* #param $model
* #return array
*/
private function _getExistingCategories( $model )
{
// Get our collection
$existingCategories = $this->_postcategory->getCollection()
->addFieldToSelect('category_id')
->addFieldToFilter('post_id', $model->getId());
// Setup our placeholder for the array of categories needed to set back on the value of the multiselect
$itemList = array();
foreach($existingCategories as $_item)
{
$itemList[] = $_item['category_id'];
}
return $itemList;
}
}
I looked back at some work I have done in Magento 1.x and I the only way I got this to work before was using javascript and the setAfterElementHtml to set the options that matched. I remember hating this method of getting it accomplished because it seemed like a work around.

Answer 1

I found the issue.
I did not have the values saved on the model.
// This is what shows it as selected on reload
$model->setData('blog_categories', $categories);
$fieldset->addField(
'blog_categories',
'multiselect',
[
'name' => 'blog_categories[]',
'label' => __('Categories'),
'title' => __('Categories'),
'required' => true,
'values' => $optionArray,
'disabled' => false
]
);