Updating a nested record with new data in elm - list

Question

I have two pieces of JSON that I've successfully decoded sequentially. I would like to take the new html_fragment and update my existing html_fragment. Generally this would be simple but my data structure is giving me difficulties:
type PostDataContainer
= PostDataContainer PostData
type alias PostData =
{ title : String
, comments : List Comment
}
type alias Comment =
{ comment_id : Int
, html_fragment : String
}
type alias CommentHtml =
{ id : Int
, html_fragment : String
}
I've just gotten CommentHtml and would like to update the existing html_fragment in Comment. This is what I have so far:
MergeCommentHtml commentHtmlData ->
case commentHtmlData of
Err err ->
Debug.log ("Error decoding CommentHtmlData" ++ toString err)
( mdl, Cmd.none )
Ok commentHtml ->
case mdl.maybePostDataContainer of
Just (PostDataContainer postData) ->
let
updatedCommentData = -- I dont know how to calculate this?
in
( { mdl | postData = { postData | comments = updatedCommentData } }, Cmd.none )
Note that commentHtml here is a List CommentHtml. Any thoughts on how to update my old comment.html_fragment with the new values in commentHtml?

Answer 1

Option 1:
just decode the data as it stands. When it's time to display it, arrange it appropriately via some function you write like rawJsonDataToNicerData.
Option 2:
Suppose you implement the following function:
-- given a new comment, and some PostData, return the new version of the PostData
updateData : CommentHtml -> PostData -> PostData
-- so now, assuming we can decode a CommentHtml with commentHtmlDeocder
-- we can do the following
dataUpdaterDecoder : Decoder (PostData -> PostData)
dataUpdaterDecoder
commentHtmlDecoder |> Decoder.andThen (\commentHtml -> updateData commentHtml)
Now wherever we were going to decode a commentHtmlDeocder we can decode a dataUpdaterDecoder instead, and use a bunch of these to update our data.
Here is an example of a relational data decoder in action using the idea above:
https://ellie-app.com/3KWmyJmMrDsa1

Answer 2

Given that commentHtmlData is a List according to a comment, I think the easiest approach is to convert it to a Dict keyed by id, then map over the existing comments looking for the comment_id in the dict. If it exists, replace html_fragment, if not then return the original unmodified:
let
commentHtmlDict =
commentHtmlData
|> List.map (\c -> (c.id, c))
|> Dict.fromList
updatedCommentData =
postData.comments
|> List.map (\comment ->
case Dict.get comment.comment_id commentHtmlDict of
Just commentHtml ->
{ comment | html_fragment = commentHtml.html_fragment }
Nothing ->
comment
)