How to marshal struct as if it were an anonymous struct?

Question

Anonymous struct fields are usually marshaled as if their inner exported fields were fields in the outer struct.

For examble:

type foo struct {
    Foo1 string `json:"foo1"`
    Foo2 string `json:"foo2"`
}

type boo struct {
    Boo1 string `json:"boo1"`
    foo
}

and I do this:

s := boo{
    Boo: "boo1",
    foo: foo{
        Foo1: "foo1",
        Foo2: "foo2",
    },
}

b, err := json.MarshalIndent(s, "", "   ")
fmt.Println(string(b))

I get this:

{
    "boo1": "boo1",
    "foo1": "foo1",
    "foo2": "foo2"
}

But how can I achieve the same result when the foo is Not an anonymous struct? Meaning:

type boo struct {
    Boo string `json:"boo"`
    Foo foo
}

And also unmarshaling the json.

user_965900 · Answer

You have to implement a custom json.Marshaler for that.

type boo struct {
    Boo1 string `json:"boo1"`
    // "-" will tell encoding/json to ignore this field during (un)marshaling
    Foo foo `json:"-"`
}

func (b boo) MarshalJSON() ([]byte, error) {
    // Declare a new type using boo's definition, this
    // "copies" boo's structure but not its behaviour,
    // i.e. B has same fields as boo, but zero methods,
    // not even MarshalJSON -- this is necessary to avoid
    // infinite recursive calls to MarshalJSON.
    type B boo

    // Declare a new type that *embeds* those structs whose
    // fields you want to be at the same level.
    type T struct {
        B
        foo
    }

    // Create an instance of the new type with its fields
    // set from the source boo instance and marshal it.
    return json.Marshal(T{B: B(b), foo: b.Foo})
}

https://play.golang.org/p/Go1w9quPkMa

A note on "anonymous"

The label anonymous as you're using it, and as it is used in the documentation you quoted, is outdated and imprecise. The proper label for a field without an explicit name is embedded.

https://golang.org/ref/spec#Struct_types

A field declared with a type but no explicit field name is called an embedded field. An embedded field must be specified as a type name T or as a pointer to a non-interface type name *T, and T itself may not be a pointer type. The unqualified type name acts as the field name.

The distinction is important because there is such a thing as "anonymous struct fields" in Go, it's used quite often but it is different from embedded fields. For example:

type T struct {
    // F is an anonymous struct field, or
    // a field of an anonymous struct type.
    // F is not embedded however.
    F struct {
        foo string
        bar int
    }
}

How to marshal struct as if it were an anonymous struct?

1 Answers