How to create a custom admin form with multiselect type in magento 2?

I created a custom module with a multiselect type input.

My InstallSchema.php file contains this for this field.

->addColumn( 'elegible_products', \Magento\Framework\DB\Ddl\Table::TYPE_TEXT, 500, ['nullable => false'], 'Products' ) 

My Main.php file looks like

$fieldset->addField( 'elegible_products', 'multiselect', [ 'label' => __('Products'), 'title' => __('Products'), 'name' => 'elegible_products', 'values' => \Ayakil\FreeGift\Block\Adminhtml\Freegifts\Grid::getValueArray4(), 'disabled' => $isElementDisabled ] ); 

This is my Grid.php file

static public function getOptionArray4() { $data_array = array(); $data_array[0] = 'a1'; $data_array[1] = 'a2'; return($data_array); } static public function getValueArray4() { $data_array = array(); foreach(\Ayakil\FreeGift\Block\Adminhtml\Freegifts\Grid::getOptionArray4() as $k => $v) { $data_array[] = array('value' => $k, 'label' => $v); } return($data_array); } 

If i tried to save the form i am getting an error like below.

{"0":"Notice: Array to string conversion in /home/muja/www/ayakil-greens/vendor/magento/framework/DB/Adapter/Pdo/Mysql.php on line 3105","1"

How can add Multiselect in custom admin form and save the data and retrieve the data here?