First (complicated) pass at extracting a list of DiscussionMessage items

chatter_serializer.ex

defmodule OmniChat.ChatterSerializer do
  use JaSerializer

  attributes [:id, :nickname]
end

discussion_message_serializer.ex

defmodule OmniChat.DiscussionMessageSerializer do
  use JaSerializer

  attributes [:content]
  has_one :chatter,
    serializer: OmniChat.ChatterSerializer,
    include: true
end

DiscussionMessage.elm

module Online.DiscussionMessage exposing (receiveCollection)

import Dict exposing (Dict)
import Json.Decode as JD
import Json.Encode as JE
import JsonApi
import JsonApi.Documents
import JsonApi.Decode
import JsonApi.Resources
import Online.Types exposing (Model, Msg, DiscussionMessage, Chatter)


receiveCollection : JE.Value -> Model -> ( Model, Cmd Msg )
receiveCollection raw model =
    { model | messages = extractMessageCollectionFromJson raw } ! []


extractMessageCollectionFromJson : JE.Value -> List DiscussionMessage
extractMessageCollectionFromJson raw =
    case JD.decodeValue JsonApi.Decode.document raw of
        Ok document ->
            extractMessageCollectionFromDocument document

        Err error ->
            Debug.crash error


extractMessageCollectionFromDocument : JsonApi.Document -> List DiscussionMessage
extractMessageCollectionFromDocument document =
    case JsonApi.Documents.primaryResourceCollection document of
        Ok resourceList ->
            List.map extractMessageFromResource resourceList

        Err error ->
            Debug.crash error


extractMessageFromResource : JsonApi.Resource -> DiscussionMessage
extractMessageFromResource messageResource =
    let
        attributes =
            JsonApi.Resources.attributes messageResource
    in
        { chatter = extractChatterAsRelatedResource messageResource
        , content = getStringAttribute "content" attributes
        }


extractChatterAsRelatedResource : JsonApi.Resource -> Chatter
extractChatterAsRelatedResource messageResource =
    let
        attributes =
            case JsonApi.Resources.relatedResource "chatter" messageResource of
                Ok chatter ->
                    JsonApi.Resources.attributes chatter

                Err error ->
                    Debug.crash error
    in
        { id = getIntAttribute "id" attributes
        , nickname = getStringAttribute "nickname" attributes
        }


getStringAttribute : String -> Dict String JD.Value -> String
getStringAttribute key dict =
    Dict.get key dict
        `Maybe.andThen` (JD.decodeValue JD.string >> Result.toMaybe)
        |> Maybe.withDefault "???"


getIntAttribute : String -> Dict String JD.Value -> Int
getIntAttribute key dict =
    Dict.get key dict
        `Maybe.andThen` (JD.decodeValue JD.int >> Result.toMaybe)
        |> Maybe.withDefault 0