TL;DR: Map relationships dynamically in a running app if you need to connect
multiple reusable ORM modules.
A longer version:
I'd often mention at various presentations that Cayenne supports generic
objects and you can create mapping in runtime. I didn't have many real-life
examples to demonstrate the need until now. But recently I encountered a good
use case that was solved by dynamic mapping - reusable ORM modules.
Say I have a reusable lib.jar containing a Cayenne project and some persistent
code built around it. Now I want to use it in app.jar (or app.war if you are
still on JavaEE). app.jar has its own Cayenne project, with entities that need
to reference entities in lib.jar. You can't relate them in the Modeler, short
of unpacking lib.jar and reassembling a new project from both projects (the
level of effort with such approach would kill most of the benefits of reuse).
Consider that in runtime EntityResolver would contain entities from both lib
and app, so all we need is to connect them. So instead of messing with XML
files, we'd create a DataChannelFilter in app.jar with "init" method that
builds all needed relationships on the fly. Now we can access these
relationships via generic DataObject API (on the app.jar side you can
optionally create regular type-safe getters and setters). I wrote a utility
relationship builder that makes this code transparent:
Relationships.oneToOne("libEntity")
.between(AppEntity.class, LibEntity.class)
.toDepPK()
.joined(AppEntity.ID_PK_COLUMN, LibEntity.ID_PK_COLUMN)
.createReverse("appEntity")
.exec(resolver);
Needless to say that all this happens in a running application, and is not
limited to relationships. E.g. you can create flattened attributes instead.
The implications are pretty exciting - you can write fully self-contained
libraries with Cayenne that can be easily extended (without unpacking) with
more tables, and otherwise integrated in app DB schemas. Dynamic mapping was
the last missing piece of a puzzle in a modular CMS design that I am working on
right now. Ironically the feature was there in Cayenne since Day 1, waiting to
get noticed.
Andrus
---------------
Andrus Adamchik
@andrus_a | @ApacheCayenne
