Hey @fils,
Sorry, no screencasts yet. But, if you look at the test queries here:
http://discuss.hypermode.com/t/list-of-test-queries/22
In general, the predicate can be a field in GraphQL. Right now, you’ve to start off specifying an _xid_, which is the external id, i.e. user-specified id for entities. Once you get results, you can also specify _uid_, which is the internally generated 64-bit integer ids.
I’ve created a Trello task to specify the offending triple when returning an error. I think you should be able to subscribe to the card to keep track of the progress. If not, let me know.