How to Parse JSON with Known and Unknown Key/Value Pairs into a Go Struct?

Unmarshal JSON with Arbitrary Key/Value Pairs to Struct

Problem

How can I parse a JSON string with known and unknown key/value pairs into a Go struct? The unknown fields can have any name and value type (string, bool, float64, or int).

Solution

Create a struct with the known fields and a slice of maps for the unknown fields:

<code class="go">type Message struct {
    Known1   string `json:"known1"`
    Known2   string `json:"known2"`
    Unknowns []map[string]interface{}
}</code>

Unmarshal the JSON string into this struct:

<code class="go">json.Unmarshal([]byte(jsonMsg), &msg)</code>

The Unknowns field will contain a list of maps representing the unknown key/value pairs.

Alternatives

1. Double Unmarshal:

   • First, unmarshal into a temporary struct containing only the known fields.
   • Then, unmarshal again into a map[string]interface{} and extract the unknown values manually.

2. Unmarshal and Type Conversion:

   • Unmarshal into a map[string]interface{}.
   • Iterate over the map and type assert the values to appropriate types.

Considerations

All three solutions are valid, but the simplest and most elegant is the initial struct-based approach. It avoids the need for additional unmarshals or manual type conversions.

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