How can I prevent JSON from being unmarshaled into a field of a struct?
from lena@gregtech.eu to golang@programming.dev on 31 Jul 2025 15:54
https://gregtech.eu/post/16712875

Marshaling the field is fine, so using the tag json:“-” won’t work. I could use the UnmarshalJSON method on the struct, but I have no idea how to implement it.

I’d rather not make a duplicate of the struct with that field removed, as it causes quite a bit of code duplication, and it’s pretty chaotic.

#golang

threaded - newest

pivot_root@lemmy.world on 31 Jul 2025 16:44 collapse

One option is to use embedded structs:

type UnmarshalStruct struct {
    Foo string
}

type MarshalStruct struct {
    UnmarshalStruct
    FieldYouDontWantUnmarshaled string
}

When marshaling, you would marshal MarshalStruct. Because UnmarshalStruct is embedded, json.Marshal automatically includes all the fields from UnmarshalStruct.

When unmarshaling, you would unmarshal UnmarshalStruct and then create a new MarshalStruct like this:

var unmarshaled UnmarshalStruct
err := json.Unmarshal(&unmarshaled)
if err != nil {
    panic(err)
}

return MarshalStruct {
    UnmarshalStruct: unmarshaled
}

Although, if I may ask: why do you not want a field to be unmarshaled? You could always just set it back to the zero value after unmarshaling, which is a lot easier of a solution.