JSON unmarshal integer field into a string

Question

I am struggling with deserializing a integer into a string struct field. The struct field is a string and is expected to be assignable from users of my library. That's why I want it to be a string, since for the purpose of writing it to the database I actually don't care about the value inside. The users can supply text, but some just assign integers.

Consider this struct:

type Test struct {
  Foo string
}

Sometimes I end up with a JSON value that is valid but won't deserialize into the struct due to the Foo field being a integer instead of a string:

{ "foo": "1" } // works
{ "foo": 1 } // doesn't

json.Unmarshal will blow up with the following error: json: cannot unmarshal number into Go struct field test.Foo of type string

See the reproduction: https://play.golang.org/p/4Qau3umaVm

Now in every other JSON library (in other languages) I have worked in so far, if the target field is a string and you get a integer the deserializer will usually just wrap the int in a string and be done with it. Can this be achieved in Go?

Since I can't really control how the data comes in I need to make json.Unmarshal unsensitive to this - the other solution would be to define Foo as interface{} which needlessly complicates my code with type assertions etc..

Any ideas on how to do this? I basically need the inverse of json:",string"

Answer 1

To handle big structs you can use embedding.

Updated to not discard possibly previously set field values.

func (t *T) UnmarshalJSON(d []byte) error {
    type T2 T // create new type with same structure as T but without its method set!
    x := struct{
        T2 // embed
        Foo json.Number `json:"foo"`
    }{T2: T2(*t)} // don't forget this, if you do and 't' already has some fields set you would lose them

    if err := json.Unmarshal(d, &x); err != nil {
        return err
    }
    *t = T(x.T2)
    t.Foo = x.Foo.String()
    return nil
}

https://play.golang.org/p/BytXCeHMvt