I suspect you might be aware of this but here goes. If one follows the book, the end of ch 9 will result in errors from the useEffect call in Venue: Started GET "/tickets.json?concert_id=undefined". The reader should be able to figure this out, but the required changes aren’t explicitly listed in the text:

wherever Venue is initiated: packs/venue_display.js didn’t work for me so I’m using packs/application.js (props no longer hard-coded and add concertId):

      <Venue
        rows={parseInt(element.dataset.rows, 10)}
        seatsInRow={parseInt(element.dataset.seatsInRow, 10)}
        concertId={parseInt(element.dataset.concertId, 10)}
      />

concerts/show.html.erb (add data attributes):

  <div id="react-element"
       data-rows="<%= @concert.venue.rows %>"
       data-seats-in-row="<%= @concert.venue.seats_per_row %>"
       data-concert-id="<%= @concert.id %>">
  </div>

Edit: I just noticed there’s bit of a naming mismatch between seats_per_row (Rails) and seatsInRow (React). I figured I’d point that out to you if you weren’t aware.