How to marshal json string to bson document for writing to MongoDB?

The gopkg.in/mgo.v2/bson package has a function called UnmarshalJSON which does exactly what you want.

The data parameter should hold you JSON string as []byte value.

 func UnmarshalJSON(data []byte, value interface{}) error

UnmarshalJSON unmarshals a JSON value that may hold non-standard syntax as defined in BSON's extended JSON specification.

Example:

var bdoc interface{}
err = bson.UnmarshalJSON([]byte(`{"id": 1,"name": "A green door","price": 12.50,"tags": ["home", "green"]}`),&bdoc)
if err != nil {
    panic(err)
}
err = c.Insert(&bdoc)

if err != nil {
    panic(err)
}

There is no longer a way to do this directly with supported libraries (e.g. the mongo-go-driver). You would need to write your own converter based on the bson spec.

mongo-go-driver has a function bson.UnmarshalExtJSON that does the job.

Here's the example:

var doc interface{}
err := bson.UnmarshalExtJSON([]byte(`{"foo":"bar"}`), true, &doc)
if err != nil {
    // handle error
}