if i were to create a custom UnmarshalJson for that data, I would create an auxiliary struct auxMain that has the same fields as the main struct but with Bar field as string. Then it unmarshals the JSON data into this auxiliary struct, extracting the Foo field and the Bar field as a string. After that, it unmarshals the Bar field as string into the Child struct, and assigns the extracted Foo field and the Child struct to the Main struct.
It's a round about way but seems to work in the playground.
func (m *Main) UnmarshalJSON(b []byte) error { type auxMain struct { Foo int `json:"foo"` Bar string `json:"bar"` } var a auxMain if err := json.Unmarshal(b, &a); err != nil { return err } var child Child if err := json.Unmarshal([]byte(a.Bar), &child); err != nil { return err } m.Foo = a.Foo m.Bar = child return nil}try it out in the PlayGround and see: https://go.dev/play/p/wWIceUxu1tj
Don't know if this is what you are looking for.
