3.4: cannot display a virtual field in a Form

In a beforeMarshall(), I create an additionnal field in my entity.
In my controller, I debug($myEntity) just before setting the data to the view, and I see my field containing the expected value.
I even created a getter like follow in myEntity.php:

protected function _getTmpImage() {
    if (isset($this->_properties['tmp_image'])) {
        return $this->_properties['tmp_image'];
    } else {
        return null;

And In my view, I see the corresponding getter’s debug() display.

But the following line:

<?= $this->Form->hidden('agpoi_images.'.$key.'.tmp_image'); ?>

desesperately creates the following html code:

<input name="agpoi_images[0][tmp_image]" class="form-control" value="" type="hidden">

Can you tell me why value is empty?

PS: I also tried to add the following statement in my entity class, even if I think I don’t have to do that according to the doc

class MyEntity extends Entity

    protected $_virtual = ['tmp_image'];


post your debug(), and Form->create()

Thanks jarekgol,

Form is created like that:

<?= $this->Form->create($agPoi, ['type' => 'file'])?>

Debug() displays:

object(ArrayObject) {
	. . . 
	agpoi_images => [
		(int) 0 => [
			'id' => '1045',
			'agpoi_id' => '736',
			'image' => '59215ec4e5f7e.jpg',
			'tmp_image' => '59215ec4e5f7e.jpg',
			'sequence' => '0',
			'copyright' => '',
			'select_audio' => '0',
			'select_time' => '01/06/2017 00:00'
	. . .

In fact if I specify the value as below, that works. So I wonder why I have to specify its proper value and why CakePHP doesn’t do it itself as for other fields in this object.

<?php foreach ($agPoi->agpoi_images as $key => $agpoi_image):?>
    <?= $this->Form->hidden('agpoi_images.'.$key.'.image'); ?> // correctly displays `image` value by itself.
    <?= $this->Form->hidden('agpoi_images.'.$key.'.tmp_image', ['value' => $agpoi_image->tmp_image]); ?>
    . . . 
<?php endif;?>

I search some of my code and found same solution

foreach($orders as $k=>$o) ... echo $this->Form->hidden($k.'.id',['value' => $o->id ]);

except that I wasn’t using virtual fields. So I can’t help you much more.
ps. in other try to build a form with many inputs, I rewrite result set from sql, so every key is an ID. It simplifies batch saving then and you don’t need to hidden(id).