无法使FindId工作（GO + MGO）

Not sure what's going on here... but I'm having a heck of a time trying to get a simple operation done. I'm new to GO (trying to switch from Node) so it's probably a Type thing...

User struct {
        ID_ bson.ObjectId `bson:"_id,omitempty" json:"_id,omitempty"`
        UTC time.Time     `bson:"utc,omitempty" json:"utc,omitempty"`
        USR string        `bson:"usr,omitempty" json:"usr,omitempty"`
        PWD string        `bson:"pwd,omitempty" json:"pwd,omitempty"`
    }
func save(w http.ResponseWriter, r *http.Request) {
  m := s.Copy()
  defer m.Close()
  user := m.DB("0").C("user")
  var a User
  json.NewDecoder(r.Body).Decode(&a)
  err := user.FindId(a.ID_)
  if err != nil {
    panic(err)
  }
}

This returns the following error

http: panic serving [::1]:53092: &{{0 0} 0xc208062600 {{0.user [{_id TE?????}] 0 0 ?
reflect.Value? 0 <nil> {?reflect.Value? ?reflect.Value? ?reflect.Value? false false [] 0}     
false []} 0.25 0}}

When I run:

a.ID_.Valid()

I get "true".

PS. I can get this to work:

func user(w http.ResponseWriter, r *http.Request) {
  m := s.Copy()
  defer m.Close()
  user := m.DB("0").C("user")
  a := &User{ID_:bson.NewObjectId(), UTC:time.Now()}
  b, _ := json.Marshal(a)
  user.Insert(a)
}

Any help would be really appreciated.

The FindId method returns a Query, not an error. Call the Query One method to get the document.

As per the docs http://godoc.org/labix.org/v2/mgo#Collection.FindId

FindId returns a Query struct which you can then call any of its functions. FindId does not return an error.

Try

var userDoc interface{}
if err := user.FindId(a.ID_).One(&userDoc); err != nil {
  panic(err)
}

You can change interface{} with whatever struct you are using for users.