Magento 2 admin form Something went wrong while saving multiselect

Question

I try to save multi select it will throw the error

Something went wrong while saving

Notice: Array to string conversion in /project/vendor/magento/framework/DB/Adapter/Pdo/Mysql.php on line 2930

class Categorylist implements ArrayInterface
{
    protected $_categoryFactory;
    protected $_categoryCollectionFactory;

    public function __construct(
        \Magento\Catalog\Model\CategoryFactory $categoryFactory,
        \Magento\Catalog\Model\ResourceModel\Category\CollectionFactory $categoryCollectionFactory
    )
    {
        $this->_categoryFactory = $categoryFactory;
        $this->_categoryCollectionFactory = $categoryCollectionFactory;
    }

    public function getCategoryCollection($isActive = true, $level = false, $sortBy = false, $pageSize = false)
    {
        $collection = $this->_categoryCollectionFactory->create();
        $collection->addAttributeToSelect('*');

        // select only active categories
        if ($isActive) {
            $collection->addIsActiveFilter();
        }

        // select categories of certain level
        if ($level) {
            $collection->addLevelFilter($level);
        }

        // sort categories by some value
        if ($sortBy) {
            $collection->addOrderField($sortBy);
        }

        // select certain number of categories
        if ($pageSize) {
            $collection->setPageSize($pageSize);
        }

        return $collection;
    }

    public function toOptionArray()
    {
        $arr = $this->_toArray();
        $ret = [];

        foreach ($arr as $key => $value)
        {
            $ret[] = [
                'value' => $key,
                'label' => $value
            ];
        }

        return $ret;
    }

    private function _toArray()
    {
        $categories = $this->getCategoryCollection(true, false, false, false);

        $catagoryList = array();
        foreach ($categories as $category)
        {
            $catagoryList[$category->getEntityId()] = __($this->_getParentName($category->getPath()) . $category->getName());
        }

        return $catagoryList;
    }

    private function _getParentName($path = '')
    {
        $parentName = '';
        $rootCats = array(1,2);

        $catTree = explode("/", $path);
        // Deleting category itself
        array_pop($catTree);

        if($catTree && (count($catTree) > count($rootCats)))
        {
            foreach ($catTree as $catId)
            {
                if(!in_array($catId, $rootCats))
                {
                    $category = $this->_categoryFactory->create()->load($catId);
                    $categoryName = $category->getName();
                    $parentName .= $categoryName . ' -> ';
                }
            }
        }

        return $parentName;
    }
}

Main.php

$objectManager = \Magento\Framework\App\ObjectManager::getInstance();
        $catConfig = $objectManager->create('Namespace\Module\Model\Module\Source\Categorylist');
        $catOptions = $catConfig->toOptionArray();

        $fieldset->addField(
            'categories',
            'multiselect',

            [
                'name' => 'categories[]',
                'label' => __('Product Categories'),
                'title' => __('Product Categories'),
                'note' => __('Add Product Categories(s), on which this cashback offer is availed.(Add ,(comma) separated Categories)'),
                'required' => false,
                 'values' => $catOptions
            ]
        );

Save.php

public function execute()
    {
        $data = $this->getRequest()->getPostValue();
        /** @var \Magento\Backend\Model\View\Result\Redirect $resultRedirect */
        $resultRedirect = $this->resultRedirectFactory->create();
        if ($data) {
            $model = $this->_objectManager->create('Namespace\ModuleName\Model\ModuleName');
            $id = $this->getRequest()->getParam('ModuleName_id');
            if ($id) {
                $curSet = $model->load($id);
            }

            $data['payment_methods'] = !empty($data['payment_methods']) ? implode(',', $data['payment_methods']) : NULL;
            $data['store_ids'] = !empty($data['store_ids']) ? implode(',', $data['store_ids']) : NULL;

            $this->_eventManager->dispatch(
                'ModuleName_prepare_save',
                ['ModuleName' => $model, 'request' => $this->getRequest()]
            );

            if (!$this->dataProcessor->validate($data)) {
                return $resultRedirect->setPath('*/*/edit', ['ModuleName_id' => $model->getId(), '_current' => true]);
            }
            $model->setData($data);
            try {               
                $model->save();
                $this->messageManager->addSuccess(__('ModuleName was saved.'));
                $this->_objectManager->get('Magento\Backend\Model\Session')->setFormData(false);
                if ($this->getRequest()->getParam('back')) {
                    return $resultRedirect->setPath('*/*/edit', ['ModuleName_id' => $model->getId(), '_current' => true]);
                }
                return $resultRedirect->setPath('*/*/');
            } catch (\Magento\Framework\Exception\LocalizedException $e) {
                $this->messageManager->addError($e->getMessage());
            } catch (\RuntimeException $e) {
                $this->messageManager->addError($e->getMessage());
            } catch (\Exception $e) {
                $this->messageManager->addException($e, __('Something went wrong while saving the ModuleName.'));
            }

            $this->_getSession()->setFormData($data);
            return $resultRedirect->setPath('*/*/edit', ['ModuleName_id' => $this->getRequest()->getParam('ModuleName_id')]);
        }
        return $resultRedirect->setPath('*/*/');
    }

Answer 1

You must convert values to a string before saving Model. Add this to your model class Namespace\ModuleName\Model\ModuleName

or you can change your post data to a string before add to model like

$data['store_ids'] = !empty($data['store_ids']) ? implode(',', $data['store_ids']) : NULL;

enter code here

public function beforeSave()
{
    $data = $this->getData('categories');
    if (is_array($data)) {
        $data = array_filter($data);
        $this->setData('categories', implode(',', $data));
    }

    return parent::beforeSave();
}