title: Getting entities by ID returns 'datastore: no such entity'
body: <p>I'm trying to pass the entity ID by URL and then get the entity of that ID.
  Here is my code.</p>
  
  <pre><code>// returns the common key for all users
  func usersKey(c appengine.Context) *datastore.Key {
      return datastore.NewKey(c, "User", "default_user", 0, nil)
  }
  
  func RegisterHandler(w http.ResponseWriter, r *http.Request) {
      c := appengine.NewContext(r)
      // ... validations bla bla
      u := User{Email: email, Hash: hash, Joined: time.Now()}
      key := datastore.NewIncompleteKey(c, "User", usersKey(c))
      if _, err := datastore.Put(c, key, &amp;u); err != nil {
          http.Error(w, err.Error(), http.StatusInternalServerError)
          return
      }
  }
  
  func ProfileHandler(w http.ResponseWriter, r *http.Request) {
      c := appengine.NewContext(r)
      params := mux.Vars(r)
  
      key := datastore.NewKey(c, "User", params["id"], 0, usersKey(c))
      u := new(User)
      if err := datastore.Get(c, key, u); err != nil {
          http.Error(w, err.Error(), http.StatusNotFound)
          return
      }
  }
  </code></pre>
  
  <p>Tthe <code>params["id"]</code> is correct. But the <code>datastore.Get</code> returns always an error: <code>datastore: no such entity</code></p>