Google Go not generating unique identifiers?

I'm trying to persist entities, and allow the database to generate a key. Unfortunately, I can't seem to tease a synthetic ID out of the API... Here's what I'm doing:

case "POST":
    d,_ := ioutil.ReadAll( r.Body)
    participant := new(Participant)
    err := json.Unmarshal( d, &participant)
    if err != nil {
        serveError( c, w, err)
        return
    }

    var key *datastore.Key;
    if participant.ID == 0 {
        // no id yet .. create an incomplete key and allow the db to create one.
        key = datastore.NewIncompleteKey( c, "participant", parentKey)
    } else {
        // we have an id.  use that to update
        key = datastore.NewKey( c, "participant", "", participant.ID, parentKey)
    }
    // PERSIST!
    putKey, e := datastore.Put( c, key, participant)
    if e != nil {
         serveError( c, w, e)
         return
    }

    // Fetch back out of the database, presumably with my new ID
    if e = datastore.Get( c, putKey, participant); e != nil {
         serveError( c, w, e)
         return
    }

    // send to the consumer
    json,_ := json.Marshal( participant)
    w.Write( json)

Well, oddly enough the ID is always zero! The details are persisted, but I get multiple values persisted with the same ID field. For grins, here's the object definition:

type Participant struct {
    ID         int64
    LastName   string
    FirstName  string
    Birthdate  string
    Email      string
    Cell       string
}

Following up with a GET operation, I am presented a list of entities (yes, the entities I created, as many times as I created them) all with the ID of zero.

[
 {"ID":0,"LastName":"Schmuck","FirstName":"Joe","Birthdate":"12-12-12","Email":"","Cell":""},
 {"ID":0,"LastName":"Schmuck","FirstName":"Jane","Birthdate":"12-12-12","Email":"","Cell":""},
 {"ID":0,"LastName":"Schmuck","FirstName":"Jane","Birthdate":"12-12-12","Email":"","Cell":""},
 {"ID":0,"LastName":"Schmuck","FirstName":"Jane","Birthdate":"12-12-12","Email":"","Cell":""},
 {"ID":0,"LastName":"Schmuck","FirstName":"Jane","Birthdate":"12-12-12","Email":"","Cell":""},
 {"ID":0,"LastName":"Schmuck","FirstName":"Jane","Birthdate":"12-12-12","Email":"","Cell":""}
]

For completeness, I've also tried using the NewKey() with an ID value of zero, but that was giving the same result. The API documentation about NewKey says something about the (string) name and ID both being zero creating an incomplete key -- I'm not sure how to make a string zero, but nil doesn't work.

Somebody help me figure out what is probably amazingly obvious once pointed to the correct section of the document.

I don't see you populate the ID field anywhere so of course it's going to be 0 since that's the zero value for that field. AppEngine won't modify your struct when it stores it. The newly created key you get back when you Put the struct contains the generated IntId() when an incomplete key is used. I think your confusion was caused by assuming something the docs never said. Namely that AppEngine would auto-populate ID fields in your stored data for you.

Since in your case you want the ID field of the struct to store the id used in the key you probably want to generate you're own unique ids for when you create a Key instead of using incomplete keys.

When the doc says a string can be zero it's referring to the zero value of a string type which would the empty string "". You can read more about zero values here: http://golang.org/ref/spec#The_zero_value

I don't see where you actually change the value of ID prior to the Put? As far as I can tell, it's always going to be 0. I would have expected to see something like:

// PERSIST!
putKey, e := datastore.Put( c, key, participant)
if e != nil {
    // ...
}
// ...
participant.ID = putKey.IntID()
putKey, e = datastore.Put(c, putKey, participant)

Not tested, but you get the picture... otherwise ID will always be 0.