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From Mike Kienenberger <mkien...@gmail.com>
Subject Re: Relationship
Date Tue, 23 Apr 2013 14:05:45 GMT
Long ago, in Cayenne 1.1, I did something similar, although I don't
know if it's now the best way to do it, or if this will completely
solve your problem.   I had defined a separate relationship using the
same db field for every object I wanted to relate to my entity.

I had a ChangeLog entity, similar to your Comments entity, and I
needed a relationship between it and every other existing entity.
Because I only needed to set the foreign key,

        aChangeLogObject.setForeign<ENTITY>Object(new<ENTITY>);

I did not create the reverse changeLogList object relationships
because I did not need them.
In the model, it looks like this.   Note that I had to create a new
relationship for each object:

==================
    <obj-entity name="ChangeLog"
className="com.xyz.cayenne.ebpp.entity.ChangeLog"
lock-type="optimistic" dbEntityName="LOG">
        <obj-attribute name="foreignRecordKey"
type="java.lang.Integer" lock="true"
db-attribute-path="FOREIGN_RECORD_KEY"/>
        <obj-attribute name="tableName" type="java.lang.String"
lock="true" db-attribute-path="TABLE_NAME"/>

[...]


    <obj-relationship name="foreignAccount" source="ChangeLog"
target="Account" db-relationship-path="foreignAccount"/>
    <obj-relationship name="foreignAddress" source="ChangeLog"
target="Address" db-relationship-path="foreignAddress"/>

[...]


    <db-relationship name="foreignAccount" source="LOG"
target="ACCOUNT" toMany="false">
        <db-attribute-pair source="FOREIGN_RECORD_KEY" target="ACCOUNT_NUMBER"/>
    </db-relationship>
   <db-relationship name="foreignAddress" source="LOG"
target="ADDRESS" toMany="false">
        <db-attribute-pair source="FOREIGN_RECORD_KEY" target="ID"/>
    </db-relationship>
==================

In my latest work project, comments were set up differently.   There
is no direct relationship between comments and other objects, either
in the database or in the code.

Instead, we have a CommentListProvider class which looks up all
comments attached to an object by an object identifier (we generally
use the object class, primary key, and application identifier).   This
allows comments to be attached to anything, and not just specific
table records.   The comment schema and supporting java code is used
by a number of applications without having any dependencies on these
applications.

Simplest use looks like..
CommentListProvider.addComment(Object, text);
List<Comment> list = CommentListProvider.getCommentList(Object);


On Tue, Apr 23, 2013 at 9:21 AM, Markus Reich
<markus.reich@markusreich.at> wrote:
> ok, e.g. I have a table comments with following columns
> ID               => Unique key of table comments
> OBJECT      => Not null, identifies the entity the comment is for
> OBJECTID   => Not null, key of the entity
> COMMENT  => Comments as VARCHAR
>
> example data
> 1  ORDER  12345  This is a comment for order 12345
> 2  ORDER  56789  This is a comment for order 56789
> 3  BILLING 11111  This is a comment for the billing 11111
>
> Now there's a table orders and a table billings (both have a unique ID
> column), so I want relationships like
> SELECT * FROM order JOIN comments ON comments.OBJECT = 'ORDER' AND
> comments.ID = order.ID
> and
> SELECT * FROM billings JOIN comments ON comments.OBJECT = 'BILLING' AND
> comments.ID = billing.ID
>
> Meex
>
>
>
>
>
>
>
>
>
> 2013/4/23 Mike Kienenberger <mkienenb@gmail.com>
>
>> I don't quite understand the question you are asking.
>>
>> Can you give us an example of what you are trying to do with your
>> comment table?   Some code using your desired mapping would be best.
>>
>>
>> On Tue, Apr 23, 2013 at 12:49 AM, Markus Reich
>> <markus.reich@markusreich.at> wrote:
>> > Hi,
>> >
>> > with the wonderful Cayenne Modeler it's possible to easily create
>> relations
>> > between DB entities.
>> > In my case I have a central table for comments, with a key specifying the
>> > content.
>> > No I need to map this key field to a constant literal, as I know it is
>> just
>> > possible to join fields with fields of the target entity :-(
>> > Is there a workaround (I don't like to make a lot of views an join them
>> ;-))
>> >
>> > regards
>> > Meex
>>
>
>
>
> --
> *Markus Reich*
> Moosbach 28/2
> 6392 St. Jakob i.H.
> www.markusreich.at / www.meeximum.at
> markus.reich@markusreich.at

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