Sub-field value as variable identifier for the next storage

Yes it is possible. However, I'm not sure which version you're using. I made some changes in the last dev. I'm not sure a did a beta-release with those. Let me check tomorrow...

I've just released a beta6. I was writing automated tests to make sure it works as expected but since you were requiring joins, I released the beta and I'll continue working on the test for the next beta.

I think I know what's going on. You're trying to map a subfield of the first storage "$.id.serial" as an id for the second. IT currently works with first-level fields. :( I'll add something to handle a JSONPath to get a field identifier value from the previous storage. Not sure when I'll have time for that but hopefully next week...

@nabcomi, are you still looking for a solution? I'll need to postpone the investigation and development of this feature. I'm working on the integration of xnttmulti into external entities module v3 as a native part of external entities. But I have other priorities before investigating that (hopefully in June or July).