how to decode a multi/single value in Json

user_1381737 0
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I want to decode a request JSON that the value of a field could be a single string or array (list).

I know how to parse if separately. but I'm looking for a way to have a dynamic decoder to parse both of them.

The value field in the JSON is the case I'm talking about

Sample for single-value:

{
    "filter":{
        "op":"IN",
        "field":"name",
        "value": "testDuplicate"
    }
}

Sample for multi-value:

{
    "filter":{
        "op":"IN",
        "field":"name",
        "value":["testDuplicate","tt"]
    }
}

Structs for single value:

type DocumentTypeSearchRequest struct {
    Filter   DocTypeFilter `json:"filter"`
}
type DocTypeFilter struct {
    Op    string `json:"op"`
    Field string `json:"field"`
    Value string `json:"value"`
}

Structs for multi-value:

type DocumentTypeSearchRequest struct {
    Filter   DocTypeFilter `json:"filter"`
}
type DocTypeFilter struct {
    Op    string `json:"op"`
    Field string `json:"field"`
    Value []string `json:"value"`
}

One solution is to try to decode with one of them if failed use another one, but I'm not sure if that is the proper way to handle this problem.

go json
1 Answers

You need a custom unmarshaler. I've talked about a similar situation in this video:

type Value []string

func (v *Value) UnmarshalJSON(p []byte) error {
    if p[0] == '[' { // First char is '[', so it's a JSON array
        s := make([]string, 0)
        err := json.Unmarshal(p, &s)
        *v = Value(s)
        return err
    }
    // else it's a simple string
    *v = make(Value, 1)
    return json.Unmarshal(p, &(*v)[0])
}

Playground link

With this definition of Value, your struct would become something like:

type DocTypeFilter struct {
    Op    string `json:"op"`
    Field string `json:"field"`
    Value Value  `json:"value"`
}

and now your Value field will always be a slice, but it may contain only a single value when your JSON input is a string, and not an array.

user_138600
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