`performReadonlyTransformation()` for `SearchableDropdownField` and `SearchableMultiDropdownField` doesn't respect lazy loading

[GuySartorelli]

A follow-up to https://github.com/silverstripe/silverstripe-framework/issues/11272

If you call performReadonlyTransformation() on a SearchableDropdownField or SearchableMultiDropdownField, it will convert the source DataList into an array, which requires pulling data for all records in the list from the database.

All that's really needed is data for the record(s) which are currently selected for the field, since it's read-only it doesn't matter what other options could have been selected, because they weren't selected.

We may need to create a new readonly formfield for these - at the very least we'll need to implement some specific logic in the performReadonlyTransformation() method to instantiate the readonly field correctly without pulling unnecessary data.

Acceptance criteria

• Calling performReadonlyTransformation() only loads data from the DB that is required to display the currently selected record(s) in the readonly form.

[johannesx75]

I'm working on it. Feel free to assign the issue to me.

Me first Idea was to overwrite performReadonlyTransformation and just change source to the selected items if the control is readonly.

Unfortunately that wont work. The value can still be set on a readonly Formfield. For example in form scaffolding the field is created and performReadonlyTransformation is run before the value is set.

So the acceptance criteria is probably missing changes to the value after performReadonlyTransformation.