As someone who uses multiple Cayenne projects in separate jars a lot (I have a
project for db documentation, another one for user management, another one for
generic object tagging etc. etc.) this is quite awesome and I have oodles of
use cases. Can you share the code for the Relationships utility class?
Allowing the Modeler to load and handle multiple models simultaneously would
also be very nice, but that’s a whole another story…
- hugi
> On 20. jan. 2017, at 01:29, Andrus Adamchik <and...@objectstyle.org> wrote:
>
> 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
>
