Map a nested JSON payload to a struct in Elixir

Question

I am attempting to port the Golang tutorial geddit to Elixir. I have done so successfully with Dartlang, but Elixir's operations on maps & lists are confusing for me.

Using HTTPoison and JSEX, I have the following code:

defmodule Redditex do
  use HTTPoison.Base

  def process_url(url) do
    "http://www.reddit.com/r/#{url}.json"
  end

  def process_response_body(body) do
    json = JSEX.decode! body
    json = Enum.map json, fn ({k, v}) -> {String.to_atom(k), v } end
    json
  end
end

My difficulty is parsing the JSON body into an appropriate struct where the JSON contains nested data. Jazz has some allusion to mapping to a structure but not with nested data.

Is there an example or a common practice to decode JSON in Elixir similar to Go's usage:

type Response struct {
Data struct {
    Children []struct {
        Data Item
    }
  }
}

type Item struct {
   Title    string
   URL      string
   Comments int `json:"num_comments"`  #mapping to another field label
}

Maybe the best way is to decode the JSON and then, when you want to work with the children, struct(Item, item_map_from_json)? — José Valim
– José Valim, Commented Aug 5, 2014 at 18:07
Struggling with that approach myself now. Newbie issues. But, I was looking for a library or built in support for building a structure based on the expected JSON response. So kinda of what you are saying at the Item level, which I am confused on how to do, but at the body level with nested structures. — Bill Christian
– Bill Christian, Commented Aug 5, 2014 at 20:07
Well, my idea was that if you need only the children, you would do something like: response["children"] |> convert_items_to_struct(). However, if you need to preserve the whole structure, you can use update_in/2 to traverse to a known nested level and update it. For example update_in response["children"], &convert_items_to_struct/1. — José Valim
– José Valim, Commented Aug 6, 2014 at 8:22

Bill Christian · Accepted Answer · 2014-08-13 22:59:25Z

5

Using the Poison JSON library I was able to get partly there for handling the nesting:

def handle_response(%{status_code: 200, body: body}) do
    json = Poison.decode!(body, as: %{"data" => %{"children" => [%{"data" => Redditex.Item}]}})
    items = Enum.map( json["data"]["children"], fn (x) -> x["data"] end )
end

Enumertion is necessary to remove the anonymous structs and the remapping of the field names has not shown as a native solution. A work path forward nonetheless.

answered Aug 13, 2014 at 22:59

Bill Christian

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