title: How to get ObjectId (_id) of my document in MGO mongo db driver for golang
body: <p>I'm working with MGO (cause I didn't found anything better that it). 
  I'have played with it and have got some result but I don't understand how to get the _id (internal Mongo  ObjectId) of document received?</p>
  
  <p>For ex:</p>
  
  <pre><code>type FunnyNumber struct {
      Value int
      _id string
  }
  
  session, err := mgo.Dial("127.0.0.1:27017")
  if err != nil {
      panic(err)
  }
  defer session.Close()
  
  // Optional. Switch the session to a monotonic behavior.
  session.SetMode(mgo.Monotonic, true)
  
  c := session.DB("m101").C("funnynumbers")
  
  funnynumber := FunnyNumber{}
  err = c.Find(bson.M{}).One(&amp;funnynumber)
  if err != nil {
      log.Fatal(err)
  }
  
  fmt.Println("Id one:", funnynumber._id)  // Nothing here? WHy? How to get it.
  fmt.Println("Value one:", funnynumber.Value)  // 62. It's OK!
  </code></pre>
  
  <p>Could someone help me, please? Ot where might I read some info about it? I haven't found anything in the MGO doc</p>
  
  <p>Schema of my document is:</p>
  
  <pre><code>{ "_id" : ObjectId("50778ce69331a280cf4bcf90"), "value" : 62 }
  </code></pre>
  
  <p>Thanks!</p>