Hi, The most important thing to remember to design your objects and their relationship is the entity group: you cannot touch (update, delete, create) entities belonging to more than 1 group in a single transaction (unless you then use transaction-bound tasks)
So, keep this point in min when you maximize the number of entity groups by keeping entities in separate groups: it will make your transactional code a little bit harder to develop if you want to maintain database integrity. regards didier On Dec 17, 7:43 pm, har_shan <harsha...@gmail.com> wrote: > i want to hear the best practice on establishing entity relationships in my > case- > > Example, for sake of discussion:- > User > Exam > Page > Question > Answer > > As you can see, from top, each entity has 1 - many relationship with entity > immediately beneath it. > Interesting case is a User can have any number of Exam and it can *scale* in > whatever manner, but a Exam usually will have few Pages, > a Page with few Questions, Questions with few Answers > > I saw this interesting post > best practice to persist medium-large > data<http://www.mail-archive.com/google-appengine-java@googlegroups.com/ms...> > > (which is also a answer to this ques) and it made me to design like this: > Also i plan to keep each entity in its own entity group, as recommended by > AppEngine - totally eliminating Parent relationship, as far as possible > > class User{ > String name; > // List<Key<Exam> exams; NOT a good idea, as this will unnecessarily load > bulk amt of Exam keys when i load User > ... > > } > > So i would use a query to get all Exams created by any User. Fine. But > > class Exam{ > String subject; > // With this i can *quickly* retrieve all Pages with keys, but as Exam and > Page are not in same Entity Group, > // how do i maintain *consistency* when, say, i delete a Page and > accordingly remove that Key entry in Exam. > // Also as said earlier, no. of pages (hence keys) shouldn't be too high, so > dont need to worry abt size > List<Key<Page> pages; > > } > > class Page{ > String pageNo. > Key<Exam> exam; // with this i can *only(?)* use query to get all Pages > under a Exam, so *unnecessary index scan*[1] > List<Key<Question> questions; > > } > > The same extends to Page -> Question -> Answer > > [1] Retrieving via Keys are faster than > querying<http://www.mail-archive.com/google-appengine@googlegroups.com/msg3384...> > > So what is your recommendation and why? > I can suspect that it should be a trade-off between unnecessary loading of > keys/extra scan of indices but want to get some thoughts and what about > consistency? > > Thanks much in advance. > p.s. though not relevant, most likely i will use low level framework, mainly > Objectify.. -- You received this message because you are subscribed to the Google Groups "Google App Engine for Java" group. To post to this group, send email to google-appengine-j...@googlegroups.com. To unsubscribe from this group, send email to google-appengine-java+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/google-appengine-java?hl=en.
