I've found that this behaviour could be related to the decoder used by BSON itself when dealing with embeded structs.

Take a look for instance at the behaviour of the following: 

https://play.golang.org/p/OBbfb_6jPjH

At the database you would expect to have

{
  "name":"Hello, playground",
  "alt_name":"This is the stuff"
}

But instead I'm having

{
  "name":"Hello, playground",
  "mystuff":{
    "alt_name":"This is the stuff"
  }
}

Which is counter-intuitive given the standard behaviour for embeded structs and json Decoding 

Is this an intended behaviour for Umarshaling/Marshaling or is this a bug?