Clojurians Log v2

When making forms in re-frame, what is the best way to have an initial value be based off a sub? For example, i have a shopping list with items having a default purchase quantity. e.g I buy eggs by the dozen. when i click on an item on the list, i want to put in how much of them i bought but have the default quantity value be the items' default purchase quantity.

right now, in my db i have a list of all items, and an "active-item" which is the id of the item i just clicked on. in my subs, then, i have item-to-buy which is the all the item details for the active-item id, taken from the list of all items. i am reading through the conduit real-world app, and they have a nice pattern for forms that's something like:

(def buy-item-form
  (let [quantity (reagent/atom 1)]
  (fn [item]
    [:form
      [input {:type "number" :value @quantity :on-change #(reset! quantity (-> % .-target .-value))}]
      ;; rest of the form...])))

I tried setting the initial atom to be based off the sub, eg:

(let [item (reframe-subscribe [::subs/item-to-buy])
      quantity (reagent/atom (:default-quantity @item)])

in context, my ideal would be when someone clicks on eggs, the quantity value is already filled to 12, but they could change it to 6 or 18 or whatever exact amount they bought at that time.

@webmaster601 make another sub

if the item map already has a default quantity assoc’d to it, you’re just going to do something like

(reg-sub ::subs/item-to-buy-default-quantity
  (fn [_qvec]
    (subscribe [::subs/item-to-buy]))
  (fn [item _qvec]
    (:default-quantity item)))

also i would say that idiom is to deref in the binding, so (let [item @(subscribe [::subs/item-to-buy])] …) rather than deref when you use it.

finally, your quantity can be in the db too! doesn’t need to be local state.

> that idiom is to deref in the binding Depends. It's not an idiom for form-2 or form-3 components.

form? are you referring to docs’ “layer”?

I'm referring to Reagent view functions.

https://github.com/reagent-project/reagent/blob/master/doc/CreatingReagentComponents.md#the-three-ways

ah i see. why is it not idiom there?

Because if you deref in the outer let then a form-2 or form-3 component will not be updated when the sub changes. There are cases when you want that behavior, but it's almost never the case.

ahh that makes sense, thanks

i guess i usually put my subscribes inside the render function

Ah, thank you for this! I think adding it to the db might be the simplest for how my app is structured. I'll give these a go!