Let me first say I have a solution to this problem, but I'm interested in knowing whether there is a better way, and whether I'm doing something wrong.
I have a table of objects on the front-end of a webapp, I need to asynchronously load some data for the objects as it is needed on a per-object basis. The server returns a JSON array containing the data for that object, and the data contains the object's key, so I can update the object on the front-end with its data. When there is no data, I just get an empty array, which unfortunately presents no way of updating the object, since I don't have the key to update it with. This can result in another query later, which is a waste of time/resources. I can't modify the server, is there a way to do this nicely?
My current solution is to just set the object's data to an empty array before sending the request, then just update when the result is received if the result is nonempty.
I was wondering if there is a better/more idiomatic way to do this.
For reference, I'm using Elm with PostgREST as the backend.
You can use currying and partial function application to indicate which object ID should be updated.
I'm assuming you have some code similar to this:
type Msg
= ...
| FetchData Int
| DataFetched [Data]
| DataFetchFail Http.Error
-- inside the update function
update msg model =
case msg of
...
FetchData id =
model ! [ Task.perform DataFetchFail DataFetched (Http.post ...) ]
If you define your DataFetched constructor to include the ID as the first parameter, you can use partial application to include the ID for future lookup, regardless of what the server returns.
Here's the same code chunks with this idea:
type Msg
= ...
| FetchData Int
| DataFetched Int [Data]
| DataFetchFail Http.Error
-- inside the update function
update msg model =
case msg of
...
FetchData id =
model ! [ Task.perform DataFetchFail (DataFetched id) (Http.post ...) ]
You could also add the ID to the Fail message for more fine-grained error messages.