Entity querying config entities does not work, so neither does JSON API collection filtering: provide helpful DX

This does not meet the definition of critical. I agree it's a very very important bug though!

Quoting \Drupal\Core\Entity\Query\QueryInterface::condition()'s docs:

   * @param $field
   *   Name of the field being queried. It must contain a field name, optionally
   *   followed by a column name. The column can be the reference property,
   *   usually "entity", for reference fields and that can be followed
   *   similarly by a field name and so on. Additionally, the target entity type
   *   can be specified by appending the ":target_entity_type_id" to "entity".
   *   Some examples:
   *   - nid
   *   - tags.value
   *   - tags
   *   - tags.entity.name
   *   - tags.entity:taxonomy_term.name
   *   - uid.entity.name
   *   - uid.entity:user.name
   *   "tags" "is the same as "tags.value" as value is the default column.
   *   If two or more conditions have the same field names they apply to the
   *   same delta within that field. In order to limit the condition to a
   *   specific item a numeric delta should be added between the field name and
   *   the column name.
   *   @code
   *   ->condition('tags.5.value', 'news')
   *   @endcode
   *   This will require condition to be satisfied on a specific delta of the
   *   field. The condition above will require the 6th value of the field to
   *   match the provided value. Further, it's possible to create a condition on
   *   the delta itself by using '%delta'. For example,
   *   @code
   *   ->condition('tags.%delta', 5)
   *   @endcode
   *   will find only entities which have at least six tags. Finally, the
   *   condition on the delta itself accompanied with a condition on the value
   *   will require the value to appear in the specific delta range. For
   *   example,
   *   @code
   *   ->condition('tags.%delta', 0, '>'))
   *   ->condition('tags.%delta.value', 'news'))
   *   @endcode
   *   will only find the "news" tag if it is not the first value. It should be
   *   noted that conditions on specific deltas and delta ranges are only
   *   supported when querying content entities.

… it doesn't say this, but AFAIK the %field.entity.%field_on_that_referenced_enitty only works for entities that have the same storage back-end. Content entities are stored in the database by default, config entities are not. So this could never work.

IOW: core's configuration entity system does not support this, so neither can JSON API. The entity query system in core should do better checking around this and provide a more informative exception though!

I was going to suggest something like is_subclass_of($this->resourceType->getDeserializationTargetClass(), ConfigEntityInterface::class); … but that only works if you're querying a config entity type directly!

Anyway, let's not wait for core to get its act together, we can improve DX today.

Failing test.

And fix.

Nice improvement. LGTM.

👏🏽

Moving to Drupal core's issue queue.

I'm working on https://www.drupal.org/project/drupal/issues/3122113