`AddUniqueSlugValidation` update script breaks on linked fields

[bakerkretzmar]

Bug Description

If a Blueprint's slug field is a linked field, e.g. a field defined in a common fieldset used in many places, the AddUniqueSlugValidation update script added in #3671 breaks.

Deep in the Blueprint class there's an array_merge() being called on the field key in the config, but on linked fields field is a string with the name of the original fieldset and field, so you get a TypeError: array_merge(): Argument #1 must be of type array, string given.

How to Reproduce

1. Have a Statamic site on a version old enough to need to run that upgrade script, e.g. 3.1.18.
2. Have a slug field in a fieldset.
3. Have a Blueprint with that slug field linked, something like this:

      - handle: slug
        field: common.slug
        config:
          required: true
          localizable: true

4. Run composer update and eventually get a TypeError.

Extra Detail

The error comes from line 470 of Statamic\Fields\Blueprint inside the ensureFieldInSectionHasConfig() method:

cms/src/Fields/Blueprint.php

Lines 466 to 472 in 9aa7a52

	// Get existing field config.
	$existingConfig = Arr::get($this->contents['sections'][$section]['fields'][$fieldKey], 'field', []);
	// Merge in new field config.
	$this->contents['sections'][$section]['fields'][$fieldKey]['field'] = array_merge($existingConfig, $config);
	return $this->resetFieldsCache();

This method is being called from ensureFieldHasConfig() in the same class, which in turn is being called from the updateBlueprint() method in the AddUniqueSlugValidation update script.

In my case, the $existingConfig argument to array_merge() ends up being 'common.slug' which causes the error.

Environment

Statamic 3.1.23 Pro (3.1.18 before running the script)
Laravel 8.47.0
PHP 8.0.7
No addons installed

Install method (choose one):

• Fresh install from statamic/statamic I think

[jasonvarga]

Ah, thanks!

For what it's worth, it's not the end of the world if that doesn't run. If you do want slugs to be unique, you can manually add a unique_entry_value:{collection},{id},{site} validation rule. (Although from how detailed this issue is, I'm sure you worked that out.)