Let's take a step back for a moment. Fundamentally, all these API layers do is map back and forth between untyped datastore Entity objects and Java classes.
Let's say the user runs a query that returns an Entity. Somehow the framework must figure out what Java class to instantiate and populate with the contents of that entity. There are only two possibilities, either: 1) The framework must know a-priori how to map Entity kinds to Java classes 2) The framework must read the full java class information out of the Entity If you go with #2, you've just created a framework that will break your datastore when you rename your Java objects. I'll assume nobody thinks this is actually a good idea. So then, how does the framework know how to map an Entity to a class? You must have *some* sort of registration process. Let's go through the options: * Registration when the framework first finds the concrete class. Unfortunately, this won't work because there is no guarantee that the framework has ever seen the class. There's no guarantee that the class is even loaded in the classloader. * Registration when the framework loads. You can do this with a classpath scanner, or a hook into the build process that generates classes. Both have downsides. * Explicit registration with a method call in your code. This is the path taken by Objectify. It is simple, reliable, and offers the ability to layer a classpath scanner on top. Jeff On Thu, Apr 1, 2010 at 12:23 AM, Yasuo Higa <higaya...@gmail.com> wrote: > Hi Jeff, > >> In other words, Twig cannot perform the simple query: >> >> Iterable<Object> foo = ofy.query().ancestor(yourobject); >> >> If you ever want to support something like this, you will need a >> registration process. >> >> If you ever want to support true polymorphic queries, you will need a >> registration process. >> > Slim3 supports kindless ancestor queries and true polymorphic queries, > and does not need a registration process. > > http://sites.google.com/site/slim3appengine/slim3-datastore/queries-and-indexes/introducing-queries > http://sites.google.com/site/slim3appengine/slim3-datastore/polymorphic-model > > Yasuo Higa > > -- > 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. > > -- 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.
