That's correct. First - you should read our documentation about entity groups. The reason you can't update parents is because the actual key of the object embeds parent information. For instance, a category key may look like this:
Category(Mammal) Subcategories may have keys that decode to the following: Category(Mammal):SubCategory(Bipedal):SubCategory(Simian) If you create the categories "Bipedal" and "Simian" first, you'll have an entity with the key: SubCategory(Bipedal):SubCategory(Simian) You can't change a key once an object is persisted. It needs to become a new entity entirely, as keys are immutable. Thus - to place these under the Category with "Mammal" as a key, you'll have to create a new object and destroy the old object. As a design pattern, generally speaking you should try to create parent objects first, as you could have an issue where deleting an old category fails after creating a new one, or vice versa. Since you won't be able to create a transaction around these objects (they won't be in the same entity group), you'll end up with either data loss or an orphaned, duplicate category. This will be rare, though, so it may not make sense to code to avoid this. On Tue, Apr 13, 2010 at 4:54 AM, gemma <gemmasilv...@googlemail.com> wrote: > Hello! > > I'm currently writing an app where the datamodel has a set of Category > objects with SubCategory children which represents a classification > tree. I've now gotten to the stage where I'm attempting to apply > these Category and SubCategory objects as attributes of an Item > object, but am getting an error that says that once an object has been > persisted, you can't create or modify its parent. > > That's cool, I'm happy to change my Item class so that instead of > having persistent Category and SubCategory items directly as > attributes they have the string repesentation of the Category and > SubCategory keys - but is this an acceptable design approach? > > Any thoughts would be greatly appreciated. > > Thanks! > Gemma > > -- > 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<google-appengine-java%2bunsubscr...@googlegroups.com> > . > For more options, visit this group at > http://groups.google.com/group/google-appengine-java?hl=en. > > -- Ikai Lan Developer Relations, Google App Engine Twitter: http://twitter.com/ikai Delicious: http://delicious.com/ikailan ---------------- Google App Engine links: Blog: http://googleappengine.blogspot.com Twitter: http://twitter.com/app_engine Reddit: http://www.reddit.com/r/appengine -- 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.
