Add a field callback right before the content is saved/collected inside the transaction during element save

[Anubarak]

I often have the requirement to ensure unique values for an element - kinda like the commerce order number.. Quite frequently I use your sequence table for this. However there is often a struggle when/where to create this unique number.

Right now there are several places where / when to create this unique value and all of them are non optimal

• on client side before the form is submitted -> really bad for obvious reasons
• \craft\services\Elements::beforeSave -> does not work, the element is validated afterwards, there is no good way to rollback the increased sequence in case other fields are invalid... of course I could run the validation by myself but there are other issues with that as well
• \craft\base\Element::beforeSave -> the very same issue as in Element::beforeSave
• \craft\base\Element::afterPropagate -> won't work because the content has to be stored as this point otherwise I would have to save the content again. I did that in Craft 4 since the content was in a separate table but with the new JSON approach it doesn't feel so good anymore

Previously I did that during
\craft\base\Field::serializeValue but that's not possible anymore due to this change

cms/src/services/Elements.php

Lines 3745 to 3748 in 2578c97

	if ($element->isNewForSite && $field->isValueEmpty($value, $element)) {
	// don't store empty values if element is new for site
	// https://github.com/craftcms/cms/issues/16797
	continue;

#16797

my field value will be empty at this point and I don't want to set it to something random.

What I basically need is a callback right before the field value is stored in order to increase the sequence value with the ability to throw an exception in case of an error so the increased sequence value will roll back as well leaving no empty values.

My only real way right now would be to return a custom value like :empty: as a field value and return false in isFieldEmpty in case the value is :empty:. However that would nearly break all usages of that field everywhere since everywhere else isFieldEmpty cannot be used anymore just to bypass this one empty check.

Thank you

[brandonkelly]

Your field type could override isValueEmpty() and just have it always return false regardless of the actual value.

Alternatively, you could define a new class which implements craft\base\Serializable and stores the existing value on a property within it, and generates a new one via its serialize() method if it’s null.

use craft\base\Serializable;

class MyGeneratedValue implements Serializable
{
    public function __construct(
        private readonly ?string $value,
    ) {
    }

    public function serialize(): string
    {
        if (!isset($this->value)) {
            // define it here
        }

        return $this->value;
    }
}

Then have your field type’s normalizeValue() method return an instance of it:

use craft\base\ElementInterface;

public function normalizeValue(mixed $value, ?ElementInterface $element): mixed
{
    if ($value instanceof MyGeneratedValue) {
        return $value;
    }
    return new MyGeneratedValue($value);
}