FIX Do not load source in SearchableDropdownTrait when lazy-loading

[emteknetnz]

Issue #11272

silverstripe/installer ci run with pr - https://github.com/emteknetnz/silverstripe-installer/actions/runs/9473993800 (includes admin behat) (green)

Note the SearchableDropDownField seems like it's broken in an elemental context, I've created a new card for this

Test setup:

MyDataObject.php

<?php

use SilverStripe\ORM\DataObject;

class MyDataObject extends DataObject
{
    private static $table_name = 'MyDataObject';

    private static $db = [
        'Title' => 'Varchar'
    ];

    private static $has_one = [
        'MySubDataObject' => MySubDataObject::class
    ];
}

MySubDataObject

<?php

use SilverStripe\ORM\DataObject;

class MySubDataObject extends DataObject
{
    private static $table_name = 'MySubDataObject';

    private static $db = [
        'Title' => 'Varchar'
    ];

    private static $has_many = [
        'MyDataObject' => MyDataObject::class
    ];
}

ModelAdmin.php

<?php

use SilverStripe\Admin\ModelAdmin;

class MyModelAdmin extends ModelAdmin
{
    private static $url_segment = 'MyModelAdmin';

    private static $menu_title = 'My model admin';

    private static $managed_models = [
        MyDataObject::class,
    ];
}

MySubModelAdmin.php

<?php

use SilverStripe\Admin\ModelAdmin;

class MySubModelAdmin extends ModelAdmin
{
    private static $url_segment = 'MySubModelAdmin';

    private static $menu_title = 'My sub admin';

    private static $managed_models = [
        MySubDataObject::class,
    ];
}

[GuySartorelli]

This might result in unexpected behaviour down the line, but there's not a lot we can do about that.

The one place that looks like this might cause problems is MultiSelectField::performReadonlyTransformation() - I think that'll need to be updated to use $this->sourceList instead of $this->getSource().

Alternatively a new getSourceList() method could be added and used in most places that currently use getSource() - that's how TagField seems to have gotten around this problem.

[emteknetnz]

Yes unfortunately it does seem to break MultiSelectField::performReadonlyTransformation() when lazy loading

I don't think changing that to use $this->sourceList will really fix this issue, because ultimately it needs to be converted to use an array anyway because in readonly mode it's using a LookField, which internally uses arrays.

To fix this it feels like we need to convert a whole bunch of things to use generators to handle very large sets of options. Given there's a workaround to use a numeric field (which was the old fallback) feels like this is more effort than it's worth, at least for now. Feels more like an API break piece of work where we also get rid of a bunch of the convoluted class hierarchy that exists for dropdowns

I'll close this PR

[GuySartorelli]

The readonly list only needs to display the items from the list which have actually been selected though, right? i.e. the current value of the field. Surely we can get a readonly formfield that displays the values without loading every single record in the DB into an array?

[emteknetnz]

Possibly

However looking at my closed PR in hindsight, it's honestly hackish. getSource() should just return the source and not care if it's lazyloaded or not