Marshal into a bson.Raw

Using gopkg.in/mgo.v2/bson, I wonder how to marshal an interface{} value into a value of type bson.Raw.

The documentation for bson.Raw states:

Using this type it is possible to unmarshal or marshal values partially.

But I can't find a Marshal function that would return bson.Raw.

What am I missing?

Example of what I try to do:

package main

import (
    "fmt"

    "gopkg.in/mgo.v2/bson"
)

func main() {
    // How to avoid a MarshalRaw help function?
    raw, err := MarshalRaw("Hello world")
    if err != nil {
        panic(err)
    }

    fmt.Printf("%+v\n", raw)
}

func MarshalRaw(v interface{}) (*bson.Raw, error) {
    bin, err := bson.Marshal(struct{ Raw interface{} }{v})
    if err != nil {
        return nil, err
    }

    var raw struct{ Raw bson.Raw }
    err = bson.Unmarshal(bin, &raw)
    if err != nil {
        return nil, err
    }

    return &raw.Raw, nil
}

Output:

&{Kind:2 Data:[12 0 0 0 72 101 108 108 111 32 119 111 114 108 100 0]}

Solution

bson.Raw is used as a value both when marshaling and unmarshaling.

To transform an interface{} into a bson.Raw, the first thing to do is to marshal it so that you get the plain document data that represents whatever is being marshaled:

    var value interface{} = bson.M{"some": "value"}
    data, err := bson.Marshal(value)
    if err != nil {
            log.Fatal(err)
    }

and then it may have one or more fields unmarshaled into bson.Raw values:

    var doc struct{ Some bson.Raw }
    err = bson.Unmarshal(data, &doc)
    if err != nil {
            log.Fatal(err)
    }

or even the entire document:

    var doc bson.Raw
    err = bson.Unmarshal(data, &doc)
    if err != nil {
            log.Fatal(err)
    }

If you want the entire document rather than just a field, you can also use this shortcut:

    doc := bson.Raw{3, data}

The 3 constant represents a document in the bson specification, and it must of course match the provided data. Since BSON only supports documents at the top level, we know this must be correct.