Ah, i missed an important piece. You are right, Didier. Making all entities to be isolated doesn't seem to work out, atleast because of this important use case, in my example:
All Pages will be in particular *order* under any Exam and similarly, all Questions will be in some *order* as well under any Page, same for Answers under Question. So i might do this with a "pageNo" field and to maintain consistency while updating this field, all Pages should be in entity group of Exam, all Questions should be same group of corresponding Page - so that i can update all in a single transaction! This means, Exam, Page, Question and Answer should all be in same entity group?! Should be ok, as this entity group will not be too big - atleast its a < a User data, as per entity group thumb rule? Or am missing something. Thanks, On Dec 18, 1:22 pm, har_shan <harsha...@gmail.com> wrote: > Yes, understood. Thanks. Am just trying my best to keep entities in > isolation unless am really forced to group them. > There is this use case in my example: > User while creating a Exam, will create, edit, delete any Page, > Question, Answer as they wish. > At regular intervals, say 1 min, i will persist all changes. At this > time, i can be successful in persisting a Page but unsuccessful in > some Question/Answer (say) > So i will just retry failed ones at next save. > I just don't want to put all Page, Question, Answer is same entity > group and fail the save altogether if a particular entity save is > failed - just to maintain integrity of each save. > Makes sense - atleast for my example? > > Also what about my relationship mentioned in my prev post? > > Thanks again. > > On Dec 18, 11:53 am, Didier Durand <durand.did...@gmail.com> wrote: > > > > > > > > > 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. 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