Go: encoding/json: no way to preserve the order of map keys

[lavalamp]

The encoding/json library apparently offers no way to decode data into a interface{} without scrambling the order of the keys in objects. Generally this isn't a correctness issue (per the spec the order isn't important), but it is super annoying to humans who may look at JSON data before and after it's been processed by a go program.

The most popular yaml library solves the problem like this: https://godoc.org/gopkg.in/yaml.v2#MapSlice

[ALTree]

This was asked in the past. Last week: #27050, and an old issue: #6244, where brad wrote:

JSON objects are unordered, just like Go maps. If you're depending on the order that a specific implementation serializes your JSON objects in, you have a bug.

While it's unlikely that the current behaviour of the json package will change, a proposal to add some new mechanism that preserves order (similar to the one in the yaml package) may be accepted. It would help, though, to have something more concrete to look at, i.e. a complete, full fledged proposal explaining what the new piece of API would look like.

[lavalamp]

My search failed to turn up either of those issues, thanks.

I tried to make it clear that this is not about correctness, it's about being nice to human readers. If that's not a relevant concern then we can close this. If there is some chance of a change being accepted then I can make a proposal.

[ALTree]

If there is some chance of a change being accepted then I can make a proposal.

I don't know if there is but the package owners can probably tell you: cc @rsc @dsnet @bradfitz

[mvdan]

There is a bit of precedent to making things nicer for human readers. For example, fmt sorts maps when printing them, to avoid random ordering in the output.

The fmt and json packages are different, and sorting keys isn't the same as keeping the original order, but I think the purpose is similar. Making the output easy to read or modify by humans.

I imagine there's no way to do this under the hood or by simply adding extra API like Encoder.SetIndent, as native maps simply don't keep the order.

Adding a type like MapSlice sounds fine to me - I'd say file a proposal unless someone else comes up with a better idea in the next few days. There's the question of whether the proposal will get past "publish this as a third-party package", but otherwise it seems like a sound idea.

[mvdan]

Similar proposal in the past - keeping the order of the headers in net/http: https://github.com/golang/go/issues/24375

Just like in this case, the big question was where to store the actual order of the map keys.

[dsnet]

Strictly speaking, you can do this today using the raw tokenizer, that json.Decoder provides. That said, I think there will be comprehensive review of all json proposals and issues in the Go1.12 cycle. I can imagine solutions for this issue that also address problems of not being able to detect duplicate keys in objects.

[lavalamp]

I think the proposal would look like a flag (.SetPreserveOrder()) on the decoder that makes a MapSlice instead of a map[string]interface{}, plus making the encoder handle that type.

@dsnet Yeah, that solves the input half of the problem, but it's very inconvenient.

[dsnet]

Alternatively, it could output as [][2]interface{}, in which case you won't need to declare a new type in the json package.

[lavalamp]

I actually like that a lot. It should probably still be declared in the json package just for documentation purposes.

[rsc]

For now it seems like the best answer is a custom type (maybe a slice of pairs) with an UnmarshalJSON method that in turn uses the tokenizer.

[Zamiell]

For the people who stumble upon this issue from Google, the following two libraries (pick one) can help you if you need an ordered JSON map:

https://gitlab.com/c0b/go-ordered-json
https://github.com/iancoleman/orderedmap

Also, for reference, see this common StackOverflow answer:
https://stackoverflow.com/questions/25182923/serialize-a-map-using-a-specific-order

Of course, it would be fantastic if this were eventually part of the standard library, so I'll eagerly await a proposal from someone more proficient than I.

[roshangade]

As per JSON specification, order does not matter.
Reference: https://tools.ietf.org/html/rfc7159#section-1

Other technologies, which use JSON, where order matters.
Example: GraphQL
Reference: https://graphql.github.io/graphql-spec/June2018/#sec-Serialized-Map-Ordering

It's a much-needed enhancement.

[astleychen]

Feature needed to simplify our implementation on ordered serialization/deserialization.

[eliben]

Strictly speaking, you can do this today using the raw tokenizer, that json.Decoder provides. That said, I think there will be comprehensive review of all json proposals and issues in the Go1.12 cycle. I can imagine solutions for this issue that also address problems of not being able to detect duplicate keys in objects.

We just ran into a case where the order of fields in a JSON file was important, and this code snippet was helpful. However, when order matters not only in the top-level JSON object but also in deeper nested objects, the need to preserve order complicates the code significantly - instead of "just" unmarshaling the object we have to do a series of piece-meal unmarshals to json.RawMessage so that we can use the unparsed byte stream at the right level.

@polinasok

[mickep76]

Using MapSlice in JSON.

type MapItem struct {
        Key, Value interface{}
}

type MapSlice []MapItem

func (ms MapSlice) MarshalJSON() ([]byte, error) {
        buf := &bytes.Buffer{}
        buf.Write([]byte{'{'})
        for i, mi := range ms {
                b, err := json.Marshal(&mi.Value)
                if err != nil {
                        return nil, err
                }
                buf.WriteString(fmt.Sprintf("%q:", fmt.Sprintf("%v", mi.Key)))
                buf.Write(b)
                if i < len(ms)-1 {
                        buf.Write([]byte{','})
                }
        }
        buf.Write([]byte{'}'})
        return buf.Bytes(), nil
}

Complete example with unmarshal in Go Playground

As a package mapslice-json.

[dfurtado]

Hi, I have a question about this issue.

I have been looking at the source code a bit and I found that the function func (me mapEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) in the encode.go actually sort the keys by doing:

sort.Slice(sv, func(i, j int) bool { return sv[i].s < sv[j].s })

Here's a bigger snippet:

// Extract and sort the keys.
keys := v.MapKeys()
sv := make([]reflectWithString, len(keys))
for i, v := range keys {
    sv[i].v = v
    if err := sv[i].resolve(); err != nil {
        e.error(fmt.Errorf("json: encoding error for type %q: %q", v.Type().String(), err.Error()))
    }
}
sort.Slice(sv, func(i, j int) bool { return sv[i].s < sv[j].s })

Why this sort is done in first place? Is there any reason or did I miss something?

I was just giving a go and removed line and the map[string]interface{} go serialized correctly.

cc @rsc @dsnet @bradfitz

Thanks!!! =)

[mvdan]

@dfurtado json encoding should be deterministic. Ranging over a map has an unspecified order, so there isn't a stable order we can use by default. So the encoder falls back to the equivalent of sort.Strings.

[andig]

I would like to pick up the discussion in the light of go2.

It seems to me one possible solution would be to enable maps to support ordered keys. This would help multiple use cases, for example with url.Values which is a map but unable to maintain the initialization order during Values() which in turn leads to invalid output for some APIs (if the API is well-designed is not point of the discussion here).

Or add an ordered map type and use it in the standard library where applicable.

I've not found a go2 proposal for maintaining map key order- would this be the point to do so?

[mvdan]

I've not found a go2 proposal for maintaining map key order- would this be the point to do so?

I would say you should use a separate issue - you could point to this issue as a potential use case, but a major language change is far more invasive than adding a feature to the json package.

[adrianre12]

I have just hit this problem too. After reading/editing/saving a json file three different applications (written in c++, C#, and Java) would not read it due to the order being changed.

[89z]

I didnt see this mentioned, so I wanted to add. If you have JSON like this:

~json
{"March": 31, "April": 30, "May": 31, "June": 30, "July": 31}
~

The key order will not be maintained. So as a workaround, you can change your
input to one of these:

~json
[["March", 31], ["April", 30], ["May", 31], ["June", 30], ["July", 31]]
[{"March": 31}, {"April": 30}, {"May": 31}, {"June": 30}, {"July": 31}]
~

then the order will be maintained. I understand this will not always be
possible, as maybe you dont have control over the input. But I wanted to mention
it.