Thank you @dblack and @zachdaniel. I started with :read but didn’t realize the error was api/json/music/artist/5 looking specifically for :id (and not the primary key). Using source to map :ArtistId to :id and this is working.
And thank you for pointing to the tests — these are terse and to the point, super clear and easy to learn from.
(4) public example: I now have ducks-in-a-row for a 2nd Primer tutorial. This starts from an sqlite c/ Livebook, passes through identities, validation, pagination, telemetry, bulk create, mermaid diagrams, and end with a minimal Phoenix app showing a json api. The Mix/Phoenix part will probably be a Github repo with sequential commits.
(This would set the stage for the last Primer, that uses AirTable-sync’ed postgres, and have auth / policies / pubsub. The trilogy should give someone new to Ash a three-hour straight path through the most important features, and let y’all answer me once instead of answering the same questions repeatedly 
)
