How Can I Dynamically Change JSON Tags in Go Structs at Runtime?

Dynamically Changing JSON Tags in Structs

Problem

A struct with nested structs is given, and the goal is to dynamically modify the JSON tag of a specific field within the struct before JSON encoding it. The desired JSON output is to have a specific field name overriden.

Solution

Using an Anonymous Struct in MarshalJSON

In Go versions 1.8 and above, a technique can be employed to dynamically change the JSON tag of a field at runtime. This involves creating an anonymous struct with the desired field tags within the MarshalJSON method of the original struct.

func (u *User) MarshalJSON() ([]byte, error) {
    type alias struct {
        ID   int64  `json:"id"`
        Name string `json:"name"` // The modified JSON tag
        tag  string `json:"-"`
        Another
    }

    var a alias = alias(*u)
    return json.Marshal(&a)
}

Here, the alias struct has the same fields as the User struct, but the Name field has the desired JSON tag ("name" instead of "first"). By returning the JSON encoding of the alias struct, the JSON field name can be overridden dynamically.

Iterating Over All Fields

To iterate over all fields of a struct, including embedded structs, use the reflect package as follows:

value := reflect.ValueOf(*u)
for i := 0; i < value.NumField(); i++ {
    tag := value.Type().Field(i).Tag.Get("json")
    field := value.Field(i)
    fmt.Println(tag, field)
}

This code will iterate over all fields, including those in the embedded Another struct, and print the JSON tag and field value for each field.

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