Insert the original.json file into a running MongoDB instance. I am using localhost:27017 for this example. That document is fetched and re-inserted after its _id is changed. I was hoping it would preserve fields with null values but it does not.

inserted.json

{
  "_id" : "456321",
  "user" : {
    "id" : 654321
  }
}

mgo_null_test.go

package main

import (
	"fmt"
	"gopkg.in/mgo.v2"
	"log"
	"os"
	"sync"
	"time"
)

func main() {
	if err := run(); err != nil {
		fmt.Fprintf(os.Stderr, "error: %v\n", err)
		os.Exit(1)
	}
}

type User struct {
	Id             *int    `json:"id" bson:"id"`
	NestedIntNull  *int    `json:"nested_int_null,omitempty" bson:"nested_int_null,omitempty"`
	NestedBoolNull *bool   `json:"nested_bool_null,omitempty" bson:"nested_bool_null,omitempty"`
	NestedStrNull  *string `json:"nested_str_null,omitempty" bson:"nested_str_null,omitempty"`
}

type Tweet struct {
	Id       string  `json:"_id" bson:"_id"`
	IntNull  *int    `json:"int_null,omitempty" bson:"int_null,omitempty"`
	BoolNull *bool   `json:"bool_null,omitempty" bson:"bool_null,omitempty"`
	StrNull  *string `json:"str_null,omitempty" bson:"str_null,omitempty"`
	User     *User   `json:"user" bson:"user"`
}

const (
	MongoDBHosts              = "localhost:27017"
	AuthDatabase              = "tweets"
	TestDatabase              = "tweets"
	ConcurrentNumberOfQueries = 1
)

func run() error {
	mongoDBDialInfo := &mgo.DialInfo{
		Addrs:    []string{MongoDBHosts},
		Timeout:  60 * time.Second,
		Database: AuthDatabase,
	}
	mongoSession, err := mgo.DialWithInfo(mongoDBDialInfo)
	if err != nil {
		log.Fatalf("CreateSession: %s\n", err)
	}
	mongoSession.SetMode(mgo.Monotonic, true)

	var waitGroup sync.WaitGroup

	waitGroup.Add(ConcurrentNumberOfQueries)
	for query := 0; query < ConcurrentNumberOfQueries; query++ {
		go RunQuery(query, &waitGroup, mongoSession)
	}
	waitGroup.Wait()
	log.Println("All Queries Completed")
	return nil
}

func RunQuery(query int, waitGroup *sync.WaitGroup, mongoSession *mgo.Session) {
	defer waitGroup.Done()
	sessionCopy := mongoSession.Copy()
	defer sessionCopy.Close()

	collection := sessionCopy.DB(TestDatabase).C("tweets")
	log.Printf("RunQuery : %d : Executing\n", query)

	var tweet Tweet
	err := collection.FindId("123456").One(&tweet) //original.json
	if err != nil {
		log.Printf("RunQuery : ERROR : %s\n", err)
		return
	}
	tweet.Id = "456321"
	err = collection.Insert(&tweet) //inserted.json
	if err != nil {
		log.Fatal(err)
	}
}

original.json

{
    "_id" : "123456",
	"int_null" : null,
	"bool_null" : null,
	"str_null" : null,
    "user" : {
      "id" : 654321,
		    "nested_int_null" : null,
    		"nested_bool_null" : null,
    		"nested_str_null" : null
    }
}