TreeDropdownField fails to validate after clearing value in react forms

[blueo]

When using a Treedropdownfield in a react form (eg the Linkfield Module SiteTreeLink) the field will pass a 'required' validation if the field is cleared - even though this should reset the field to an 'empty' state.

Steps to reproduce:

1. open a FormBuilder Form with a required TreeDropdownField - eg the Link Field module silverstripe/linkfield
2. Try submitting the form without any changes - observe the field will be correctly marked as 'required'
3. Add a value to the TreeDropdownField
4. Clear the TreeDropdownField using the 'x' symbol
5. submit the form - observe that even though the field is 'empty' it will now pass validation

Investigation
it seems that when clearing the field it gets set to a value of 0 https://github.com/silverstripe/silverstripe-admin/blob/2/client/src/components/TreeDropdownField/TreeDropdownField.js#L425 which at some point gets converted to a string.

This means that when Validator.js checks the value of the field it sees '0' and this passes the 'required' validation rule as it is a non-empty string:

by contrast, when the field loads - the value is ""

Seems we should find another way to clear the field to an empty string instead of 0.
If I update SINGLE_EMPTY_VALUE to equal "" then the validation works as expected

this was tested on v 4.13.1 of the admin module - unsure if this is still a problem in v5 at this stage.

Acceptance criteria

• A common empty value is defined. (probably an empty string)
• The default value when instantiating the field is the common empty value.
• When you clear a pre-existing value, the field is reset the the common empty value.

PR

• FIX TreeDropdownField fails validation if it is empty #1578

[GuySartorelli]

this was tested on v 4.13.1 of the admin module - unsure if this is still a problem in v5 at this stage

Just to clarify, you have a link to code in the 2 branch, which is specifically CMS 5 code. Can you please update your link if it's incorrect, or else update the description to indicate that both CMS 4 and 5 are affected, if both are affected?

[GuySartorelli]

I don't want to test this with linkfield as that's not a supported module and I'm not familiar with its code.

open a FormBuilder Form with a required TreeDropdownField

Can you please provide me with reproduction instructions for setting this up without using linkfield? Could be in asset-admin or possible elemental inline-editable block if that uses the formbuilder.

[sabina-talipova]

I confirm, this is the issue in CMS4 and CMS5 as well.
Replacing SINGLE_EMPTY_VALUE = 0 to SINGLE_EMPTY_VALUE = '' fix this issue and TreeDropdownField fails validation if it's required and empty.

[blueo]

ah great thanks @sabina-talipova I was having some trouble getting it going in another place!

[mfendeksilverstripe]

Relates to silverstripe/silverstripe-elemental#329

[maxime-rainville]

We spent half an hour arguing about what the proper empty value should be and coludn't reach a sensible decision. We'll create a SPIKE to review the option and wider implications.