In Go, how to Unmarshal bson byte[] data into an array of structs?

Question

What's the best way to Unmarshal bson byte[] data into an array of structs, when the array of structs is passed into an interface{} parameter?

For demonstration purposes, in the following code, I use bson.Marshal() on the inStructArr to get the byte[] type of data. This is so I can use bson.Unmarshal(...) to pipe into the outStructArr.

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

type User struct {
    Name string
}

func DecodeArrData(inStructArr, outStructArr interface{}) {
    inStructArrData, _ := bson.Marshal(inStructArr)
    bson.Unmarshal(inStructArrData, outStructArr) // <-- Error happens here
    // What's the right way of accomplishing this?
}

func Main() {
    outUsers := &[]User{}
    inUsers := []User{User{"A"}, User{"B"}}

    DecodeArrData(inUsers, outUsers)
}

When I do this, the error-message I get is: Unsupported document type for unmarshalling: []User. What's the right way of doing this?

Thanks in advance!

Answer 1

The Marshal and Unmarshal functions work with BSON documents, not BSON arrays.

Wrap the slices in a struct to provide the document expected by the package:

func DecodeArrData(inStructArr, outStructArr interface{}) error {
    in := struct{ Data interface{} }{Data: inStructArr}
    inStructArrData, err := bson.Marshal(in)
    if err != nil {
        return err
    }
    var out struct{ Data bson.Raw }
    if err := bson.Unmarshal(inStructArrData, &out); err != nil {
        return err
    }
    return out.Data.Unmarshal(outStructArr)
}

If you are willing to take advantage of an undocumented feature of the Marshal function and add some BSON format knowledge to your application, then you can omit the wrapper.

The undocumented feature of Marshal is that it encodes slices as BSON arrays. The BSON array can be decoded using a bson.Raw value with Kind set the the BSON code for arrays (the value 4) and Data set to the array data:

func DecodeArrData(inStructArr, outStructArr interface{}) error {
    inStructArrData, err := bson.Marshal(inStructArr)
    if err != nil {
        return err
    }
    raw := bson.Raw{Kind: 4, Data: inStructArrData}
    return raw.Unmarshal(outStructArr)
}