Objectify and email as #Id - google-app-engine

Question

I am not sure if I use the #Id in objectify the right way.
Right now I am using the eMail-Address as #Id field. The email field will be set on the server-side only (OAuthService.getCurrentUser.getEmail)
First question: Is this a good idea?
If I create for example an Item-class which has RegistrationTO as it's parent does it make sense to use the email-address as the #Id field in my Item-class or should Item-class have it's own, auto-generated, id and Key parent to specify the relation?
Objectify-Tutorial recommends to avoid #Parent - so, here I think it's not necessary either.
I am right?
Here my RegistrationTO:
public class RegistrationTO implements Serializable {
private static final long serialVersionUID = 1L;
#NotNull
#Size(min = 5, max = 20)
private String firstname;
#NotNull
#Size(min = 5, max = 20)
private String name;
#NotNull
#Size(min = 5, max = 20)
private String country;
#Id
#NotNull
#Size(min = 5, max = 20)
#Pattern(regexp = "\b[A-Z0-9._%-]+#[A-Z0-9.-]+\\.[A-Z]{2,4}\b")
private String email;
public RegistrationTO() {
}
public RegistrationTO(final String firstname, final String name, final String company) {
this.firstname = firstname;
this.name = name;
this.country = country;
email = "will be set on server (Oauth)";
}
public String getFirstname() {
return firstname;
}
public String getName() {
return name;
}
public String getCountry() {
return country;
}
public String getEmail() {
return email;
}
public void setEmail(final String email) {
this.email = email;
}
}
Sample for Item class:
public class Item implements Serializable {
private static final long serialVersionUID = 1L;
#Id
Long id
//or
//#Id
//String email
Key<RegistrationTO> parent;
String itemno;
}
Thank you in advance!

Answer 1

Regarding your question if the use of e-mail as #Id is correct or not, since the email will uniquely identify each object of the class, then you are fine!
Now, regarding the #Id of your Item class, if the email uniquely identifies each object, then there is no need to create a new auto-generated Long as #Id. In general, the criterion for the selection of the #Id is to uniquely identify all the objects of the class.
For the relationship between RegistrationTO and Item classes, use the #Parent annotation only if you need these entities to be the same entity group. The code for this:
#Parent
Key<RegistrationTO> parent;
Otherwise, use a "plain" relationship (as you have it in your example) that allows RegistrationTO and Item entities to be stored in different entity groups in the GAE datastore. For more information about entity groups, take a look at:
http://code.google.com/appengine/docs/java/datastore/entities.html#Entity_Groups_and_Ancestor_Paths
Hope that helps!