ENH Add polymorphic has_one support to eager loading #11203

[beerbohmdo]

Description

Polymorphic has_one relations are currently not supported by eager-loading it just throws an error that DataObject is not a valid data class.

This PR relies on my previous PR https://github.com/silverstripe/silverstripe-framework/pull/11185, which is currently only in 5.2

Manual testing steps

<?php

use SilverStripe\ORM\DataObject;
use SilverStripe\Security\Member;
use SilverStripe\Security\Group;

/**
 * @property string|null $Title
 * @property int $PolymorphID
 * @property string $PolymorphClass
 * @method DataObject Polymorph()
 */
class MyModel extends DataObject
{
    /**
     * @var array
     * @config
     */
    private static $db = [
        'Title' => 'Varchar(255)',
    ];

    /**
     * @var array
     * @config
     */
    private static $has_one = [
        'Polymorph' => DataObject::class,
    ];
}

class MyModelController extends Controller
{
    public function index(): void
    {
        $o = MyModel::create();
        $o->Title = 'Foo';
        $o->PolymorphClass = Member::class;
        $o->PolymorphID = 1;
        $o->write();

        $o = MyModel::create();
        $o->Title = 'Bar';
        $o->PolymorphClass = Group::class;
        $o->PolymorphID = 1;
        $o->write();

        foreach (MyModel::get()->eagerLoad('Polymorph') as $item) {
            var_dump($item->Polymorph());
        }
    }
}

Issues

• 11203

Pull request checklist

• [x] The target branch is correct
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• [x] The commit messages follow our commit message guidelines
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• [x] Code changes follow our coding conventions
• [x] This change is covered with tests (or tests aren't necessary for this change)
• [ ] Any relevant User Help/Developer documentation is updated; for impactful changes, information is added to the changelog for the intended release
• [x] CI is green

[GuySartorelli]

Thanks for implementing this. Can you please include some manual testing steps? Preferably with some code I can just copy into a project and see it work. That will make this a lot easier to test and therefore I'll be able to do so sooner.

[beerbohmdo]

I've added a code snippet