How do I find a JSON object using a nested expression?

Question

From the example JSON below, I would like to return the target.id value of an object where the source.id == 'x'.

So where source.id == 'startId' return target.id == '3eecd840-e6a8-423c-a892-7df9646fde5d'.

{
      "line":[
         {
            "type":"link",
            "source":{
               "id":"startId",
               "port":"out"
            },
            "target":{
               "id":"3eecd840-e6a8-423c-a892-7df9646fde5d",
               "port":"in"
            },
            "id":"87d88a26-3a28-4db0-8016-71c1c4665f14"

         },
         {
            "type":"link",
            "source":{
               "id":"3eecd840-e6a8-423c-a892-7df9646fde5d",
               "port":"outYes"
            },
            "target":{
               "id":"49940c02-70f2-4c53-ab50-9cbf96903600",
               "port":"in"
            },
            "id":"9f8c365e-9ca7-440f-a722-c4f340782c0c"
         }
      ]
   }

I've tried JSONPath, but I cannot work out the expression to use. $.line[*].source.id gives me a list of source id's and $.line[?(@.source.id=='startId')] returns an error.

I also understand that I could iterate through each object in code, but It wouldn't be very efficient if I have tens or hundreds of 'lines' to work through. If possible I would like a more 'direct' path to the object.

I'm using javascript to code the rest of the app, so javascript examples would be helpful (with or without JSONPath).

Any reason you can't just parse the JSON and then work with the resulting object graph? (Which isn't JSON, it's an object graph. JSON is a textual notation.) From your question, it sounds like you're working with JSON as text... — T.J. Crowder
– T.J. Crowder, Commented Dec 12, 2014 at 15:42
Despite its name, JSONPath still seems to work with the parsed object, not the JSON string. So it also iterates over the object. — Felix Kling
– Felix Kling, Commented Dec 12, 2014 at 16:03
Yes I start off with JSON as a string and then create an object using JSON.parse(x). JSONPath allows me to slice json up , but doesnt seem to allow me to pick an object value based on another object value, ie. pick the 'target' where the 'source' = foo — Magic Lava
– Magic Lava, Commented Dec 12, 2014 at 16:36

Amit Joki · Accepted Answer · 2014-12-12 15:42:39Z

1

If you're getting json as string, then use var json = JSON.parse(jsonStr). Then you can do it with Array.filter

var result = json.line.filter(function(obj){
   return obj.source.id == "startId"
});

Then you could get the values like this

var ids = result.map(function(o){ return o.target.id });

answered Dec 12, 2014 at 15:42

Amit Joki

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2 Comments

T.J. Crowder Over a year ago

"I also understand that I could iterate through each object in code, but It wouldn't be very efficient..." (and that's assuming they've parsed the JSON).

Magic Lava Over a year ago

Hi Amit. Yes that works thank you, but as T.J pointed out, the filter is iterating through every 'line' and returning true or false. If this is the only way to do what I want, then I'm happy to use it. I just thought there may be a "better" way of doing it.

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