How to Unmarshal JSON with Unexpected Field Names into a Go Struct?

Unmarshaling JSON with Unknown Fieldnames to Struct

Question:

Is it possible to unmarshal JSON with fieldnames that are unknown in advance, while maintaining the given structure?

Problem:

Consider a JSON response where unknown fieldnames wrap a common structure. This structure must be mapped to a struct, but the fieldnames vary.

{
    "unknown_field": {
            "known_field_1": [[1,2,3,4,5],[10,20,30,40,50],[100,200,300,400,500]],
            "known_field_2": [[11,21,31,41,51]],
            "known_field_3": [[12,22,32,42,52],[14,44,34,44,54]]
        }
}

Answer:

Yes, it is possible to unmarshal such JSON using a map as the root struct element, where keys are the unknown fieldnames and values are instances of the known struct.

type mData struct {
    KnownField1 [][5]int `json:"known_field_1"`
    KnownField2 [][5]int `json:"known_field_2"`
    KnownField3 [][5]int `json:"known_field_3"`
}

var data map[string]mData
if err := json.Unmarshal(body, &data); err != nil {
    panic(err)
}

Output:

map[unknown_field:{[[1 2 3 4 5] [10 20 30 40 50] [100 200 300 400 500]] [[11 21 31 41 51]] [[12 22 32 42 52] [14 44 34 44 54]]}]
unknown_field {[[1 2 3 4 5] [10 20 30 40 50] [100 200 300 400 500]] [[11 21 31 41 51]] [[12 22 32 42 52] [14 44 34 44 54]]}

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