I think the value of difficulty being passed to submit-tx is a string, when your schema says it should be a set of keywords. you can throw in a print call to make sure, though I think it'll work if you pass the value to keyword and remove the :set bit from your schema

🙏, yep :set was the problem, it passes without it, both as a keyword and a string . Have I got the wrong idea about :set ? I assumed it prevented duplication in the db .eg .."Tetris", :game/difficulty :hard :hard but perhaps it's only meant to prevent duplication at the transaction level. At the document level, how might you handle a game update where there could be the potential for duplicated difficulty levels? Or similarly a favorite list where multiple game id's could appear in a vector but no more than once?

Can a single game have multiple difficulties? If so I probably gave you the wrong advice. (I was assuming a :game document means a specific instance of a game, e.g. "alice and bob are now playing yahtzee", vs. the more general concept of "yahtzee" itself--I'll blame it on skimming your message from my phone earlier 🙂) If a game can have multiple difficulties, than your schema was right to begin with, and you can instead update the submit-tx call to wrap the value in a set:

(defn new-game [{:keys [params] :as req}]
  (biff/submit-tx req
                  [{:db/doc-type :game
                    :xt/id (random-uuid)
                    :game/name (:name params)
                    :game/difficulty (set (keyword (:difficulty params)))}])})

If you're updating an existing document and you want to insert another value into a set, you can also use :db/union -- see https://biffweb.com/docs/reference/transactions/ for an example

Out of curiosity--if a game can have multiple difficulties, is game-form intended to provide multiple select? It's currently just a single-select, right?

not sure if this helps, but the latest main from malli can coerce homogenous :enum values from strings to keywords, symbols and numbers: https://github.com/metosin/malli/pull/782

You had it right the first time, each game should have a single difficulty 😄. Apologies, my follow-up question isn't easy to follow. :game/difficulty should compare the difficulties between the concept of a game not an instance:

.. {:game/name "chess", :game/difficulty :hard}
.. {:game/name "monopoly", :game/difficulty :easy}

...
  (biff/submit-tx req
    [{:db/op :update
      :db/doc-type :game
      :xt/id (:parse uuid)
      :game/difficulty (:difficulty params)}])
  ...)

[:select
      {:name "difficulty"}
      [:option {:value :easy} "easy"]
      [:option {:value :medium} "medium"]
      [:option {:value :hard} "hard"]]

re: coercion--that's handy, I wasn't aware of that!

that update handler will overwrite the difficulty value, so there will only be a single value. the document won't have multiple values unless you explicitly pass in a vector/set/some other type of collection

I wonder if you're thinking about this in an entity-attribute-value centric way, similar to how datomic works? this is an example of how XT's document-centric approach differs--when you submit a document it completely overwrites the entire previous document; it doesn't add individual facts accumulatively. (feel free to ignore if this just confuses things more!)

Thinking in terms of small updates rather than overwrites was exactly my confusion. I'll give the biff and XTDB transaction docs a good read later. Thanks