First-level many-many doesn't create snapshots

[unclecheese]

Steps to reproduce

class MyPhotoPage extends Page
{
    private static $many_many = [
        'Photos' => [
            'from' => 'MyPhotoPage',
            'to' => 'Image',
            'through' => MyPhotoPageImageLink::class,
        ],
    ];

    private static $owns = [
        'Photos',
    ];
}

class MyPhotoPageImageLink extends DataObject
{
    private static $extensions = [
        Versioned::class,
    ];

    private static $owns = [
        'Image',
    ];

    private static $has_one = [
        'MyPhotoPage' => MyPhotoPage::class,
        'Image' => Image::class,
    ];
}

• Add images
• Save draft
• Observe that no "images added" snapshots have been created

Cause

There are two orphaned snapshots that are created here:

• A snapshot that was created for the image on upload (no ownership at write time)
• A snapshot that was created when the page was saved and the link was created (MyPhotoPageImageLink created). This should be populated.

The latter snapshot is empty due to this line:

https://github.com/silverstripe/silverstripe-versioned-snapshots/blob/master/src/SnapshotPublishable.php#L645

where linking objects go up the ownership chain of their parents, no the child ,which now has a populated ownership chain. The way this is written, it just tries to create a snapshot for MyPhotoPage, which is empty as it is the top of the chain.

I'm sure it was written this way for a reason -- probably for objects one level deeper?

But either way, this is a really common use case and it's broken.

[kinglozzer]

I have a simple document -> tags many-many-through: https://gist.github.com/kinglozzer/03ced85925047a094f7bcd0633634cf6

If I create a document like:

Title: tagged foo
Tags: foo

Then update it to:

Title: tagged foo and bar
Tags: foo, bar

When I revert back to the first published version, the title is reverted but the bar tag isn’t removed. Is this issue why that doesn’t happen? Just wanted to check before I open an issue 😅