How to Unmarshal JSON with Dynamic Keys in Go?

Dynamic Key Handling in Viper/JSON Unmarshaling

When dealing with JSON data that deviates from a predetermined format, unmarshalling in Go can be challenging. Consider a JSON configuration file with dynamic keys, such as the following:

{
  "things" :{
    "123abc" :{
      "key1": "anything",
      "key2" : "more"
    },
    "456xyz" :{
      "key1": "anything2",
      "key2" : "more2"
    },
    "blah" :{
      "key1": "anything3",
      "key2" : "more3"
    }
  }
}

To unmarshal this data into a Go struct with dynamic keys, one approach is to utilize a map:

type X struct {
    Things map[string]Thing
}

type Thing struct {
    Key1 string
    Key2 string
}

By using map[string]Thing, the resulting struct can handle any number of dynamic keys. Unmarshal the data using:

var x X
if err := json.Unmarshal(data, &x); err != nil {
    // handle error
}

This approach allows for straightforward unmarshalling of dynamic keys in both JSON and Viper (using viper.Get("things") to obtain an array of Thing values).

However, if the key itself must be a member of the struct, a post-unmarshalling loop can be employed to update the fields:

type Thing struct {
    Name string `json:"-"` // add the field
    Key1 string
    Key2 string
}

// Fix the name field after unmarshal
for k, t := range x.Things {
    t.Name = k
    x.Things[k] = t
}

This allows the key to be incorporated into the struct while preserving the original dynamic nature of the JSON data.

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