google app engine - When do you store ids and when do you store keys in gae datastore? -

suppose datastore model looks this:

@entity public class user {      @id     private long id;     @index     private string email;     private long dateofbirth;     // more fields... }  @entity public class topic {      @id     private long id;      private string topictitle;     private long date;   }  @entity public class comment {      @id     private long id;     @parent     private key<topic> topickey;     private long commenterid;     private string text;     private long date;  } 

where entity comment has parent entity topic. know 1 should store keys when specifying @parent such did in comment entity, should 1 store key of commenterid? or storing long id of user enough?

just wondering best practice storing references other entities when not parents - should store id , generate key later or store key entity. there reason why might 1 on other?

edit: since using cloud endpoints, responses appengine project json. parameterized type of key not allowed in client libs. me, id can work , key<?> can work. note should return websafe version of client using:

mykey.getstring(); 

typically there no reason store key reference. keys take more space - both in datastore, , in objects transfer , client.

using key may necessary if same entity kind can either or child of entity. technically possible, , data models can use approach, although rare use case.

nb: use id of parent in objects - same reason (less space). in datastore entities parent id can extracted child entity key. use low-level datastore api, - need check how correctly annotate child-parent relationship in library use.