Can't update to-one relationship based on public id

[andrus]

Given:

• Entity A with ID hidden from the Agrest model, and a special public ID attribute used instead.
• Entity B with to-one relationship to A

Issue: how do we update B's "a" value? The following update results in an exception. The error happens even if "B.a" was excluded from the B model via a constraint (I suppose it will not throw if exclusion happens via entity overlay).

PUT {\"a\":val}

Caused by: java.lang.IllegalArgumentException: Entity 'A' has unexpected number of ID attributes: 0
at io.agrest.runtime.protocol.EntityUpdateJsonTraverser.converter(EntityUpdateJsonTraverser.java:158)
at io.agrest.runtime.protocol.EntityUpdateJsonTraverser.processRelationship(EntityUpdateJsonTraverser.java:107)
at io.agrest.runtime.protocol.EntityUpdateJsonTraverser.processObject(EntityUpdateJsonTraverser.java:81)
at io.agrest.runtime.protocol.EntityUpdateJsonTraverser.traverse(EntityUpdateJsonTraverser.java:39)
at io.agrest.runtime.protocol.EntityUpdateParser.parse(EntityUpdateParser.java:45)
at io.agrest.runtime.protocol.EntityUpdateParser.parse(EntityUpdateParser.java:34)
at io.agrest.provider.EntityUpdateReaderProcessor.read(EntityUpdateReaderProcessor.java:24)
at io.agrest.provider.EntityUpdateReader.readFrom(EntityUpdateReader.java:51)
at io.agrest.provider.EntityUpdateReader.readFrom(EntityUpdateReader.java:29)
at org.glassfish.jersey.message.internal.ReaderInterceptorExecutor$TerminalReaderInterceptor.invokeReadFrom