Unable to unselect all multiselect values

[Vinai]

Magento version: 2.1.2

It is not possible to unselect and save all multiselect attribute values for a product.
After the save completes the state of the product attribute before the product was saved is unchanged.
It seems the problem is that the JS does not submit multiselect form elements with no selected values, at least on the server side completely unselected multiselect attribtues are not part of the $_POST['product'] array.

The exact same issue existed for years on M1 but was finally fixed there. It seems like this issue was introduced again for M2.

Preconditions

None special.

Steps to reproduce

1. Create a custom multiselect attribute. Ot just install the sample data (e.g. the issue can be reproduced using the features_bag or strap_bags attributes).
2. Open the product in the admin interface.
3. Select one or more values of the multiselect attribtue.
4. Save
5. Unselect all values of the multiselect attribute
6. Save again

Expected result

The product attribute should have no value for the product.

Actual result

The previously selected product attributes still are selected.

[Vinai]

@fooman gave the input that probably the can_be_empty property has to be set on the \Magento\Framework\Data\Form\Element\Multiselect instance.
For system configuration multiselect that can be done by adding the <can_be_empty> node to the system.xml file, but in case of EAV attributes I see no obvious way to do so since they are DB based.

[Vinai]

On a side note, the method \Magento\Framework\Data\Form\Element\Multiselect::getElementHtml() adds the CSS class admin__control-multiselect (amongst others). Should the class not also be usable on the store front?

[Vinai]

More info in case it's useful: the property can_be_empty would be set in \Magento\Backend\Block\Widget\Form::_applyTypeSpecificConfig() but that method is never called. There are 3 classes that would trigger the flow of control to enter that method, but none of three classes are used on the edit product form. Is this M1 legacy code that wasn't deleted?

\Magento\Catalog\Block\Adminhtml\Product\Edit\Action\Attribute\Tab\Attributes::_prepareForm()
\Magento\Catalog\Block\Adminhtml\Product\Attribute\NewAttribute\Product\Attributes::_prepareForm()
\Magento\Catalog\Block\Adminhtml\Product\Edit\Tab\Attributes::_prepareForm()

[ivanweiler]

It seems that same problem exists with configuration multiselects. For example
Stores>Configuration>General>Top destinations - not possible to unselect all and save in 2.1.2.

[antboiko]

Hi @Vinai , thanks for reporting this issue. We have internal ticket MAGETWO-57607 to track this issue on our side.

Best,
Anton.

[pravalitera]

It happens also on custom attributes with custom sources. I have put Magento\Eav\Model\Entity\Attribute\Backend\ArrayBackend as backend_model .

When i save in backoffice without options, i have a string in the before save with the previous options selected.
When i save in backoffice with one/more options, i have an array.

[erikhansen]

@antboiko Any update on this issue? A Magento Enterprise merchant I'm working with is experiencing this issue and they're looking for a patch. Before emailing support for a patch, I wanted to see if this was fixed in develop so that I can grab a patch?