Decoding Multi Level Json

tushar

Below is the json I get after encoding it using Jason

"{\"topic\": \"dummy_topic\", \"msg\": \"{\"fruit\":\"apple\",\"dish\":\"apple-pie\"}\"}"

I am trying to pass it as the body of the request while calling an API using HTTPoison , it gives me the json decode error

tried the same on terminal and I get below ,

iex(10)> var = "{\"topic\": \"dummy_topic\", \"msg\": \"{\"fruit\":\"apple\",\"dish\":\"apple-pie\"}\"}"
"{\"topic\": \"dummy_topic\", \"msg\": \"{\"fruit\":\"apple\",\"dish\":\"apple-pie\"}\"}"
iex(11)> Jason.decode(var)                                                                              
{:error,
 %Jason.DecodeError{
   data: "{\"topic\": \"dummy_topic\", \"msg\": \"{\"fruit\":\"apple\",\"dish\":\"apple-pie\"}\"}",
   position: 35,
   token: nil
 }}

Which is the exact same error I get when when I use HTTPoison ,

my expectation is I should be getting something like below when I use Jason.decode ,

%{
  "topic" => "dummy_topic",
  "msg" => %{"fruit" => "apple", "dish" => "apple-pie"}
}

Do you know where I am getting it wrong or what can be done to get the expected output ?

9 comments

#json #jason #elixir-httpoison #multi-level-json #decode

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2020-07-09 07:00:49 UTC

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NobbZ

Due to the doublequotes in the value of msg beeing improperly escaped, you don’t have valid JSON.

Post #1

ityonemo

It’s in position 35, as the error states.

Post #2

NobbZ

"{\"topic\": \"dummy_topic\", \"msg\": \"{\"fruit\":\"apple\",\"dish\":\"apple-pie\"}\"}"
                                          ^^

That is where the error is, this matches exactly with what I said, the quotes aren’t escaped properly. The following should be valid:

~S'{"topic": "dummy_topic","msg":"{\"fruit\":\"apple\",\"dish\":\"apple-pie\"}"}'

Alternatively, you have to double escape in the msg field:

"{\"topic\": \"dummy_topic\", \"msg\": \"{\\\"fruit\\\":\\\"apple\\\",\\\"dish\\\":\\\"apple-pie\\\"}\"}"

I prefer the sigil approach for readability.

Post #3

tushar

Thanks @NobbZ ,

It was happening because of the sigil , I found it finally and solved it by replacing the body of the inner json into a String that would contain a place holder for it.

So my requirement here was the value of a key in the map would be another json , that json would also be derived from a map , I used Jason.encode to do this , it puts an escape \" before the starting braces ,
and then after composing the body of the final json I used sigil (~s) which had put another escape prefixing the escape , which was causing a problem during Jason.decode since it was unable to find the start of the inner map.

How I fixed it is in the final json I just put a placeholder <body> and then replaced the body with the Jason.encode inner json string.

With sigils is there a way to avoid escaping the escape characters to construct the right json that can be used flawlessly with Jason.decode

Post #4

NobbZ

Jason.encode(%{
  topic: "dummy",
  msg: Jason.encode(%{a: "message blob"})
})

# or

~s'{"topic":"dummy","msg":#{Jason.encode(Jason.encode(%{a: "message blob"}))}}'

These variants should both work. I’m not sure about exact quoting here, so it might be, that you need to wrap the interpolated value in another pair of quotes.

I prefer the first approach for readability.

Post #5

tushar

"{\"topic\": \"dummy_topic\", \"msg\": \"{\"fruit\":\"apple\",\"dish\":\"apple-pie\"}\"}" → This does not work

iex(1)> Jason.decode("{\"topic\": \"dummy_topic\", \"msg\": \"{\"fruit\":\"apple\",\"dish\":\"apple-pie\"}\"}")
{:error,
 %Jason.DecodeError{
   data: "{\"topic\": \"dummy_topic\", \"msg\": \"{\"fruit\":\"apple\",\"dish\":\"apple-pie\"}\"}",
   position: 35,
   token: nil
 }}

"{\"topic\": \"dummy_topic\", \"msg\": {\"fruit\":\"apple\",\"dish\":\"apple-pie\"}}" → This works

iex(2)> Jason.decode("{\"topic\": \"dummy_topic\", \"msg\": {\"fruit\":\"apple\",\"dish\":\"apple-pie\"}}")
{:ok,
 %{
   "msg" => %{"dish" => "apple-pie", "fruit" => "apple"},
   "topic" => "dummy_topic"
 }}

The only difference between the strings is

"{\"topic\": \"dummy_topic\", \"msg\": *\"*{\"fruit\":\"apple\",\"dish\":\"apple-pie\"}*\"*}"

The escape sequences causing problem are wrapped in *

Those two escape sequences if removed , makes it work fine.

Is there a way with Sigils I can choose to not have those escapes ? or I have to do string manipulation to have it working ?

Post #6

NobbZ

I thought you want msg to be a string containing JSON? Then of course you can not remove the wrapping quotes that make it a string.

Post #7

tushar

True but as is , the string causes a problem when I try to decode it using Jason.decode

Its unable to find the inner map… , any way we can counter that problem and have my desired structure back from the json string , if you need the original Elixir map which I converted to Json and and want to have it back , let me know.

Post #8