Delegate relation query logic to each field when evaluating ->relatedTo

[nateiler]

The ElementQuery::relatedTo is a powerful (and universal) way to build query relationships via a field, but most of logic is hard coded outside the control of a field type. Granted a field type can modify the query using the FieldInterface::modifyElementsQuery() method, but there is a lot of logic within the ElementRelationParamParser class and things get ugly when there's that much copy->paste to mimic the same logic.

What if the relatedTo condition logic was delegated to the field type? Looking at vendor/craftcms/cms/src/elements/db/ElementRelationParamParser.php:256 we're already iterating against the fields ... could we create and call a new method that would handle the logic?

I remember discussing this a while back. We're currently working on a couple field types that are similar to Matrix where they have child fields that can be related against.

[brandonkelly]

Definitely open to this (anything to simplify ElementRelationParamParser!).