golang mongodb (mgo) is not inserting docs

Im having issues with persisting a golang struct in mongodb using mgo.

type AN_Track_Log struct {
	Id                       bson.ObjectId `bson:"_id,omitempty"`
	user_session_id_str      string       `bson:"user_session_id_str"`

	googleanaly_pixel_id_str string `bson:"googleanaly_pixel_id_str"`
	perfaud_pixel_id_str     string `bson:"perfaud_pixel_id_str"`
	site_id_str              string `bson:"site_id_str"`
	metric_str               string `bson:"metric_str"`
	value_str                string `bson:"value_str"`
	event_str                string `bson:"event_str"`
	location_id_str          string `bson:"location_id_str"`
	referer_str              string `bson:"referer_str"`
	track_origin_str         string `bson:"track_origin_str"`
	fingerprint_str          string `bson:"fingerprint_str"`
    ...
}

p_track_log.Id = bson.NewObjectId()
err := p_mongodb_coll.Insert(&p_track_log)

the problem is that when the Insert() call completes, the only thing thats persisted in the DB is an empty doc

{u'_id': ObjectId('561809d20037873154000003')}

I check that the struct fields are indeed set, and not empty.
Any ideas as to why this is happening.
Hints are appreciated thank you

You need to export the fields by starting the field name with a capital letter.

type AN_Track_Log struct {
  Id                       bson.ObjectId `bson:"_id,omitempty"`
  User_session_id_str      string       `bson:"user_session_id_str"`

  Googleanaly_pixel_id_str string `bson:"googleanaly_pixel_id_str"`
  ...
}