How to handle null values in json response in Go using json.NewDecoder

Question

This is my struct:

type Media struct {
    MediaID   string `json:"media_id"`
    MediaType float64    `json:"media_type"`
    ViewCount float64    `json:"view_count"`
}

I'm writing an API to serve some media files and I'm using json.NewDecoder to decode and work with the response. I populate my struct like this way:

media := Media{
                MediaID:   media["media_id"].(string),
                MediaType: media["media_type"].(float64),
                ViewCount: media["view_count"].(float64),
}

It works fine in many situations but the problem is that sometimes view_count is null and it gives me this error:

interface conversion: interface {} is nil, not float64

I really appreciate any comments!

This is all the code before trying to extract response fields:

var (
    r  map[string]interface{}
)

var buf bytes.Buffer

query := map[string]interface{}{
    "query": map[string]interface{}{
        "match": map[string]interface{}{
            "content": params["q"],
        },
    },
}

if err := json.NewEncoder(&buf).Encode(query); err != nil {
    log.Fatalf("Error encoding query: %s", err)
}

res, err = es.Search(
    es.Search.WithContext(context.Background()),
    es.Search.WithIndex("myindex"),
    es.Search.WithBody(&buf),
    es.Search.WithTrackTotalHits(true),
    es.Search.WithPretty(),
)

if err := json.NewDecoder(res.Body).Decode(&r); err != nil {
    log.Fatalf("Error parsing the response body: %s", err)
}

Are you decoding the JSON to the media? If so, why not decode directly to the struct? To answer the question, use a two-value type assertion and handle the false value. — user5728991
– user5728991, Commented Jul 5, 2020 at 17:59
I'm decoding the response to map[string]interface{}. Then I try to populate Media struct like this. Is there any get(value or None) solution like in python?? — mrasoolmirza
– mrasoolmirza, Commented Jul 5, 2020 at 18:09
Is there a reason why you do not decode directly to the struct? — user5728991
– user5728991, Commented Jul 5, 2020 at 18:30
It is not necessary to declare a struct field for every key in the JSON object. It might be helpful to show some more of the code. If the JSON is decoded to map[string]interface{} and you have no control over that, then this question is not about type assertions and not JSON. — user5728991
– user5728991, Commented Jul 5, 2020 at 19:01
Decode to the struct directly: var media Media; if err := json.NewDecoder(res.Body).Decode(&media); err != nil { /* handle error */} — user5728991
– user5728991, Commented Jul 5, 2020 at 19:52

Diako Sharifi · Accepted Answer · 2020-07-05 18:06:51Z

2

If you have nil for some fields, solution is POINTER!

this code is enough if you use binding

type Media struct {
    MediaID   string `json:"media_id"`
    MediaType float64    `json:"media_type"`
    ViewCount *float64    `json:"view_count"`
}

but if you wanna assign it field by field (which is not a good idea) before assigning you should check if media["view_count"] is exist or not

var viewCount float64
tmpViewCount, ok := madia["view_count"]
if ok {
   viewCount = tmpViewCount.(float64)
}

media := Media{
   MediaID:   media["media_id"].(string),
   MediaType: media["media_type"].(float64),
   ViewCount: viewCount,
}

edited Jul 5, 2020 at 18:06

answered Jul 5, 2020 at 18:00

Diako Sharifi

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6 Comments

mrasoolmirza Over a year ago

When I use your method it gives me this error: interface conversion: interface {} is float64, not *float64

Diako Sharifi Over a year ago

my method is two different part, erase * from float64.

mrasoolmirza Over a year ago

I'm looking for something like your first method but it doesn't work and gives me the above error. your second solution works fine.

Diako Sharifi Over a year ago

@Neo if you working with JSON you can try json.Unmarshal ednsquare.com/story/…

mrasoolmirza Over a year ago

I found out that for stream application like in my case, it's better to use NewDecoder rather than Unmarshal

|

RelectoDetecto · Accepted Answer · 2020-07-06 05:01:47Z

The numeric fields in Media should probably be integer values instead of floating point values:

type Media struct {
    MediaID   string `json:"media_id"`
    MediaType int    `json:"media_type"`
    ViewCount int    `json:"view_count"`
}

Use pointers if the numeric fields if the JSON document uses null for these values. Hopefully that's not the case because the application must write a bit of extra code to deference the pointers.

type Media struct {
    MediaID   string `json:"media_id"`
    MediaType *int    `json:"media_type"`
    ViewCount *int    `json:"view_count"`
}

Either way, decode directly to Media as suggested in the question's commentary:

var media Media
if err := json.NewDecoder(res.Body).Decode(&media); err != nil {
    log.Fatalf("Error parsing the response body: %s", err)
}

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