common-issues.md

Frequently Encountered Issues

WriteXXX can only write while positioned on a Element or Value but is positioned on a TopLevel

The bson.Marshal function requires a parameter that can be decoded into a BSON Document, i.e. a primitive.D. Therefore the error message

WriteXXX can only write while positioned on a Element or Value but is positioned on a TopLevel

occurs when the input to bson.Marshal is something other than a BSON Document. Examples of this occurrence include

• WriteString: the input into bson.Marshal is a string
• WriteNull: the input into bson.Marshal is null
• WriteInt32: the input into bson.Marshal is an integer

Many CRUD operations in the Go Driver use bson.Marshal under the hood, so it's possible to encounter this particular error without directly attempting to encode data. For example, when using a sort on FindOneAndUpdate this error can occur when not properly initializing the sort variable:

var sort bson.D // this is nil and will result in a WriteNull error
opts := options.FindOneAndUpdate().SetSort(sort)
update := bson.D{{"$inc", bson.D{{"x", 1}}}}
sr := coll.FindOneAndUpdate(ctx, bson.D{}, update)
if err := sr.Err(); err != nil {
	log.Fatalf("error getting single result: %v", err)
}

The above example is resolved by initializing the sort variable:

sort := bson.D{}

Convert BSON Document to JSON

There are a variety of marshalers that can be used to encode a BSON document as JSON, including MarshalExtJSON:

doc := bson.D{{"x", 1}}

jsonBytes, err := bson.MarshalExtJSON(doc, true, false)
if err != nil {
	log.Fatalf("error encoding json: %v", err)
}

m := make(map[string]interface{})
if err := json.Unmarshal(jsonBytes, &m); err != nil {
	log.Fatalf("error decoding json: %v", err)
}
fmt.Printf("json: %v\n", m)