Using mongodb go driver for decoding documents into structs with custom type fields

Question

I'm a beginner in both go and mongodb. I try to decode a DocumentResult into a struct using bson tags, and it does not work for a custom type wrapping a string. Can it be done without changing the field's type to a string?

    import (
    "context"
    "github.com/mongodb/mongo-go-driver/mongo"
)

type MyDoc struct {
    SomeInt int `bson:"some_int"`
    SomeString string `bson:"some_string,omitempty"`
    CustomType MyType `bson:"custom_type,omitempty"`
}

type MyType string

const myType MyType = "ABCD"

func main() {

    //Connect to db
    client, _ := mongo.Connect(context.Background(), "mongodb://localhost:27017", nil)
    db := client.Database("example_db")
    collection := db.Collection("col")

    //Insert document
    docToInsert := MyDoc{42, "The Answer", myType}
    collection.InsertOne(nil, docToInsert)

    //Retrieve document
    filterDoc := MyDoc{SomeInt: 42}
    resultDoc := &MyDoc{}
    result := collection.FindOne(nil, filterDoc)
    result.Decode(resultDoc)

    println(resultDoc.SomeInt, resultDoc.SomeString, resultDoc.CustomType)

PRINTED RESULT: "42 The Answer" //"ABCD" is missing

Thanks in advance

Answer 1

Foreword: Custom types having string as their underlying types are now handled automatically by the driver. This answer predates the driver 1.x versions where this was necessary.

---

Unfortunately you're out of luck. The current state of the official mongo go driver does not support unmarshaling string values from BSON to a Go value whose type is a custom type having string as its underlying type. This may change in the future, but for now this is not supported.

The way decoding into a struct field is handled is implemented in bson/decode.go, currently line #387:

case 0x2:
    str := v.StringValue()
    switch containerType {
    case tString, tEmpty:
        val = reflect.ValueOf(str)
    case tJSONNumber:
        _, err := strconv.ParseFloat(str, 64)
        if err != nil {
            return val, err
        }
        val = reflect.ValueOf(str).Convert(tJSONNumber)

    case tURL:
        u, err := url.Parse(str)
        if err != nil {
            return val, err
        }
        val = reflect.ValueOf(u).Elem()
    default:
        return val, nil
    }

0x02 is the BSON string type. It is only attempted to decode into the struct field if the struct field's type is any of the following: string, interface{}, json.Number or url.URL (or a pointer to these).

Unfortunately implementing bson.Unmarshaler on your custom type does not help either, as it is not checked in case of struct fields, only if the struct itself implements it. But implementing on the struct itself, you would have to duplicate the struct with the field being one of the above listed supported types (or use a map or a bson.Document type).

This is a serious limitation on the library's part which can very easily be solved, so let's hope for the best that they add support for this in the near future.