How to Unmarshal a JSON Object into a Struct with a Map of Slices?

Custom Unmarshaling a Struct into a Map of Slices

Problem

Custom unmarshaling is required to decode JSON into a struct with a map of slices. Using the default behavior, the map remains empty.

Solution

Using a Custom Unmarshaler:

1. Implement the json.Unmarshaler interface for the target struct.

2. In the UnmarshalJSON method:

   • Unmarshal the keys and values of the JSON object as raw, non-decoded JSON.
   • Extract the Last field from the raw JSON value, if it exists.
   • Convert the raw values into a map of slices and assign it to the Pair field.

Alternative Solution (Not Using a Map):

If the JSON structure is fixed, the target struct can be adjusted to match the JSON layout without using a map.

Example:

Custom unmarshaling code using the custom UnmarshalJSON method:

<code class="go">func (r *OHLC_RESS) UnmarshalJSON(d []byte) error {
    // Decode keys and values
    var obj map[string]json.RawMessage
    if err := json.Unmarshal(d, &obj); err != nil {
        return err
    }

    // Extract "last" field
    if last, ok := obj["last"]; ok {
        if err := json.Unmarshal(last, &r.Last); err != nil {
            return err
        }
        delete(obj, "last")
    }

    // Decode and assign Pair field
    r.Pair = make(map[string][]Candles, len(obj))
    for key, val := range obj {
        cc := []Candles{}
        if err := json.Unmarshal(val, &cc); err != nil {
            return err
        }
        r.Pair[key] = cc
    }
    return nil
}</code>

Alternative solution by adjusting the struct:

<code class="go">type OHLC_RESS struct {
    Pair []Candles `json:"XXBTZUSD"`
    Last int64     `json:"last"`
}</code>

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