Apache Cayenne: user-defined tables: examples?

Question

I\'m looking at Apache Cayenne as my ORM layer for a new application.

Part of my DB model will be defined in a fixed model which will be known at compile time.

Answer 1

Yes, Cayenne generic objects make it possible to alter your model in runtime. You have an additional challenge to update the actual schema. There are a few unknowns in the description (is the underlying DB shared by multiple apps/users; concurrency of the schema changes; do the changes need to be preserved in the underlying Cayenne model after they are applied). But here is a rough idea of how I would approach this:

Whenever a user is ready to make new changes, load the affected cayenne-project.xml separately from ServerRuntime using cayenne-project.jar library.

Injector i = DIBootstrap.createInjector(new CayenneProjectModule());
ProjectLoader loader = i.getInstance(ProjectLoader.class);
Project p = loader.loadProject(new URLResource(..));

Make changes:

ConfigurationNodeVisitor mapChangeAction = .. // implement this to make your changes
p.getRootNode().acceptVisitor(mapChangeAction);

Save back to file system:

i.getInstance(ProjectSaver.class).save(p);

Now you might create a second ServerRuntime using a fresh model, and then run migrations against DB using API from org.apache.cayenne.merge package. You can create MergeToken's based on the changes you've made above using DbAdapter.mergerFactory() and then execute them:

DataDomain domain = newRuntime.getDataDomain();
DataNode node = domain.getDataNode("nodename");
DataMap map = domain.getDataMap("mapname");

List<MergerToken> tokens = ...
MergerContext mergerContext = new ExecutingMergerContext(map, node);
for (MergerToken tok : tokens) {
    tok.execute(mergerContext);
}

Finally replace your original ServerRuntime that had the old model, with the 'newRuntime'.