field_field_config_presave expects there to be a 'handler' setting but there isn't

Yeah, kind of ugly :-)

The thing is, several e_r code in Core assume all ER fields have a Selection handler - DER bypasses the notion of "selection handler" completely ? How does it handle "what is referenceable and what is not" ?

Also, on that regard, maybe you guys would have a thing to say about the validation strategy considered in #2577963: Let entity_ref Selection handlers be in charge of the field validation (since it proposes to rely more heavily on the Selection handlers in the ValidReference constraint) ?

We store the same settings as ER but keyed by entity type.
Something like

      'node' => [
        'handler' => "default",
        'handler_settings' => [],
      ],
      'user' => [
        'handler' => "default",
        'handler_settings' => [],
      ],

We don't use ValidReference constraint we have ValidDynamicReference constraint.

@larowlan I committed the patch but head still has some fails. :(

I see, makes sense.

Still, the fact that the 'handler' setting expected from the parent is not present, is a bit worrying. Could it be left present but set to NULL ?

We need a core followup for #7 and perhaps DER followup for #1978714: Entity reference doesn't update its field settings when referenced entity bundles are deleted.