After sleeping on it, I feel I'm missing what add-epoch is supposed to do, and how index-by is supposed to use it. I'd welcome clarification.

Thanks for this PR! Didn't have a chance to look at the changes.

To clarify add-epoch it is converting dates to unix timestamps so that when we display the posts (the initially downloaded ones 10 or 20) they are displayed in order. index-by adds this prop when it converts [] to {} so later it's trivial to navigate.

When we get the blog posts from the API one of the properties is - :createdAt "2021-05-20T11:31:13.731Z" and we use this information to add additional property :epoch 1621510273731. Then in the subscription we sort them by :epoch so they appear in order --

Take a look at the re-frame-10x-demo. Press CTRL + h, if the re-frame 10x panel is not visible. There when you expand articles you will see :epoch along :createdAt.

Hope that helps

Ah! After a few iterations, I think this works:

(defn add-epoch
  "Add :epoch timestamp based on :createdAt field."
  [item]
  (assoc item :epoch (-> item :createdAt rdr/parse-timestamp .getTime)))

(defn index-by
  "Index collection by function f (usually a keyword) as a map"
  [f coll]
  (into {}
        (map (fn [item]
               (let [item (add-epoch item)]
                 [(f item) item])))
        coll))

I added it to the PR when you can take a look. Thanks!

Thanks so much for this! Great updates across the board!