NEW: Support dot syntax in form field names

[sminnee]

This change adds support for these in a few places.

• Form::saveInto($record)
• Form::loadDataForm($record)
• Form::loadDataForm($_POST)

Fixes #9163

To do

• Documentation
• Tests

[sminnee]

Making as WIP as the tests need to be fixed, new tests need to be written, and docs

[emteknetnz]

This pull request hasn't had any activity for a while. Are you going to be doing further work on it, or would you prefer to close it now?

[sminnee]

I'm using this code in production via composer-patches so I'm definitely keen to see it make its way into mainline. I'll endeavour to get tests added and tests passing to make it mergeworthy in the next few weeks.

[chillu]

@sminnee Still keen on adding tests here?

[chillu]

I've put this through its paces, wrote some unit tests, and docs with extensive code examples that I've tested manually as well. Found a few functional gaps, e.g. the ability to get the field via fieldByName(). Note that the API overlaps between two use cases now: Using dots for nested CompositeField (e.g. TabSet), and using dots for relationship saving. One natural limitation is that you can't do both:

new FieldSet([
  new TextField('My.Field'),
  new TabSet('My', [
      new TextField('Field'),
  ])
]);

I think that's acceptable, and very unlikely to happen in the real world.

I've also considered the security aspects of this, and can't fault it. You'll need to opt-in to this behaviour by whitelisting the relationships which are allowed to be written. There's some sensitivity about explicitly checking can*() permissions on the nested relationships, and being careful about create vs. update (since DataObject->getComponent() will auto-create a non-existent relationship). This is clearly flagged in the tutorial.

@emteknetnz @dhensby @sminnee Good to merge?

Marginally related gotcha: Some unrelated FormTest failures caused by #9948.

[chillu]

Alright got a thumbs up from a core committer (Dan), and the work is semi peer reviewed, so I'm calling this done! Just added another commit with a changelog entry to raise the visibility of the tutorial.