>Partitioning syntax
Correct. Not currently supported. Planned for the future of HDBDD - a general roundout of the missing SQL Syntax. In other words, its not there yet but we absolutely plan to add it in the future to HDBDD.
>Foreign keys (this i guess is possible, not sure how, read something in the dev guide on using River like associations - it will be great if you can confirm)
Yes you can create associations. Those associations can be used within views that are defined within the same HDBDD. No these associations do not generate database constraints. They also are not currently considered by either XSODATA or automatically in XSJS or SQLScript. You would have to code logic at these levels to enforce the associations as constraints. Today they are mostly for simplified view creation. In the near future they will be exposed to XSJS via an SAP supplied library and will be reusable in XSODATA and will therefore be enforced in CUD operations.
The recommendation is still to use the repository and the built in lifecycle. This would mean coding your logic at a higher level until we have the above mentioned functionality. Or you could generate constraints via SQL on first run of an application to avoid having to have manual SQL setup scripts and/or a more complicated lifecycle management approach.