Is `Enum.replace_at` authentic for updating socket.assigns?

Daniel_Xu_For · February 8, 2022, 3:26pm

Hi, Everyone:

I’m currently working on a project and I decided to give liveview a try. I’m so happy with it and thanks for all the contributions everyone made.

I’m posting to ask whether my approach is authentic. Here we go: (it’s an auction system)

data model

item: item has many bids
bid: bid belongs to an item

scenario

on the liveview page: I loaded all the items. (I’m not using temporary assign because I need to find a specific item later on)

when I click a specific item, a modal is popup, and I can create a new bid.
After the new bid is saved in the database, I want the bid number to be updated on the page.

I try to solve this with pubsub. After creating a bid, message is broadcasted to all subscriber including the liveview page.

In handle_info, I reload the bid association of the specific item. Then I use Enum.replace_at to replace that item in the items assign in the socket.

question

I knew that if I used temporary assign, I can simply [item | items] to let liveview track the difference and replace specific item. what can I do to utilize the similar mechanism without using temporary assign?
is it possible to use temporary assign in my use case? ( I need to find the detailed information of specific item after the initial rendering)

Thanks!

cmo · February 8, 2022, 9:48pm

Doesn’t your bid have anid that you can use to locate and update it?

lud · February 8, 2022, 11:06pm

I would say just like @cmo, if your item has an id then you can just do

socket = put_in(socket.assigns.item_bids[item_id], bid).

cmo · February 8, 2022, 11:32pm

Yes, but I only ever use assign or update to modify the socket. There is more going on under the covers than just putting values into the assigns map.

github.com

phoenixframework/phoenix_live_view/blob/30ee942b3a18a9e2e1f222a76a707bfba7bd94f7/lib/phoenix_live_view/utils.ex#L43

      
        
            def force_assign(assigns, nil, key, val), do: Map.put(assigns, key, val)
            
            
def force_assign(assigns, changed, key, val) do
              # If the current value is a map, we store it in changed so
              # we can perform nested change tracking. Also note the use
              # of put_new is important. We want to keep the original value
              # from assigns and not any intermediate ones that may appear.
              current_val = Map.get(assigns, key)
              changed_val = if is_map(current_val), do: current_val, else: true
              changed = Map.put_new(changed, key, changed_val)
              Map.put(%{assigns | __changed__: changed}, key, val)
            end
            
            
@doc """
            Clears the changes from the socket assigns.
            """
            def clear_changed(%Socket{private: private, assigns: assigns} = socket) do
              temporary = Map.get(private, :temporary_assigns, %{})
            
            
  %Socket{
                socket

Daniel_Xu_For · February 9, 2022, 2:46am

Thanks for the reply, I was wondering whether it’s possible to use [item| items] + prepend/append without temporary assign.
After experimenting a bit, it seems to be impossible.

Daniel_Xu_For · February 9, 2022, 2:52am

data structure

[ 
%Item{}, 
%Item{}, 
           
%item{   bids: [%bid{}, %bid{}]  } 
]

I don’t think I can use put_in or replace_at because I need to traverse the list and find the specific item.
Right now, I solve it by reloading all items as a whole, but there’s room for optimization.

I can use Enum.map to return an updated list, but it still needs to traverse the whole list.

Sebb · February 9, 2022, 6:56am

If its possible to index the collection it is easier to update:

def index_collection!(collection, key) do
  Enum.map(collection, fn item -> {Map.fetch!(item, key), item} end) |> Enum.into(%{})
end

> items = [%Item{id: 1, ...}, %Item{id: 2, ...}]
> index_collection!(items, :id)
%{1 =>  %Item{id: 1, ...}, ... }

But you lose order, be aware of that.

Daniel_Xu_For · February 9, 2022, 11:44am

order is a must-have criteria. I think I will have to traverse the list. Also, I found that Enum.reject is used during deletion in liveview doc. I guess it’s ok when the list is not huge when pagination is involved.