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From: Eric Schneider <eric.j.schneider@gmail.com>
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Date: Thu, 19 Jan 2017 20:33:54 -0500
Subject: Re: Dynamic mapping
Message-Id: <45CA900F-60BB-4308-8850-3466F9B52EB4@gmail.com>
References: <7D7E4AAC-1BBB-4A5D-A034-2969CB4A8A62@objectstyle.org>
In-Reply-To: <7D7E4AAC-1BBB-4A5D-A034-2969CB4A8A62@objectstyle.org>
To: user@cayenne.apache.org
archived-at: Fri, 20 Jan 2017 01:34:01 -0000

I'm glad somebody finally noticed it.  :-)

> On Jan 19, 2017, at 8:29 PM, Andrus Adamchik <andrus@objectstyle.org> wrot=
e:
>=20
> TL;DR: Map relationships dynamically in a running app if you need to conne=
ct multiple reusable ORM modules.
>=20
> A longer version:
>=20
> I'd often mention at various presentations that Cayenne supports generic o=
bjects and you can create mapping in runtime. I didn't have many real-life e=
xamples to demonstrate the need until now. But recently I encountered a good=
 use case that was solved by dynamic mapping - reusable ORM modules.=20
>=20
> Say I have a reusable lib.jar containing a Cayenne project and some persis=
tent code built around it. Now I want to use it in app.jar (or app.war if yo=
u are still on JavaEE). app.jar has its own Cayenne project, with entities t=
hat need to reference entities in lib.jar. You can't relate them in the Mode=
ler, short of unpacking lib.jar and reassembling a new project from both pro=
jects (the level of effort with such approach would kill most of the benefit=
s of reuse).
>=20
> Consider that in runtime EntityResolver would contain entities from both l=
ib and app, so all we need is to connect them. So instead of messing with XM=
L files, we'd create a DataChannelFilter in app.jar with "init" method that b=
uilds all needed relationships on the fly. Now we can access these relations=
hips via generic DataObject API (on the app.jar side you can optionally crea=
te regular type-safe getters and setters). I wrote a utility relationship bu=
ilder that makes this code transparent:
>=20
> Relationships.oneToOne("libEntity")
>        .between(AppEntity.class, LibEntity.class)
>        .toDepPK()
>        .joined(AppEntity.ID_PK_COLUMN, LibEntity.ID_PK_COLUMN)
>        .createReverse("appEntity")
>        .exec(resolver);
>=20
> 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.=

>=20
> The implications are pretty exciting - you can write fully self-contained l=
ibraries with Cayenne that can be easily extended (without unpacking) with m=
ore tables, and otherwise integrated in app DB schemas. Dynamic mapping was t=
he last missing piece of a puzzle in a modular CMS design that I am working o=
n right now. Ironically the feature was there in Cayenne since Day 1, waitin=
g to get noticed.
>=20
> Andrus
>=20
> ---------------
> Andrus Adamchik
> @andrus_a | @ApacheCayenne
>=20