在GAE中，什么时候用户可能没有ID

I am building a web app with go and GAE. I would like to use Google Accounts for authentication. The appengine/user package contains a type, User. I was planning on using ID property of User as the ancestor to descendent entities in the Datastore. However I'm confused by the comments in this section of documentation:

type User struct {
    Email      string
    AuthDomain string
    Admin      bool

    // ID is the unique permanent ID of the user.
    // It is populated if the Email is associated
    // with a Google account, or empty otherwise.
    ID string

    FederatedIdentity string
    FederatedProvider string
}

Source: https://cloud.google.com/appengine/docs/go/users/reference#User

Under what circumstances might an email not be associated with a google account and therefore ID be empty?

I'm very new to go and GAE so please excuse my ignorance.

There are several key differences between email and id. E.g. "The app can also access a user ID that identifies the user uniquely, even if the user changes the email address for her account." Also "Every user has the same user ID for all App Engine applications."

Like ThunderCat said, if you use a Federated Login (OpenID was the only one supported, but is no longer), then you will not get a user id.

See the docs for more info.