Beginner question about JDO best practice

Dear all,

this is the first time I'm using JDO/ORM (the JPOX implementation) and
so have a question about best practice.

Let's assume we have a class "Tag" with the only property "String
name" and according setters and getters. The related table has two
columns "PRIMARY BIGINT tag_id" and "UNIQUE VARCHAR name". Note that
the column "name" is unique.

Let's further assume that we manually create some Tag objects during
application runtime. We now want to persist these objects but we don't
care if they are already persisted in the database. If this is the
case, JDO should just attach the object to the database instance. What
is the best practice to accommodate this? Do I have to manually check
every object for existence first? I would like to avoid this.

Let's say the tag "coffee" is already available in the database. I'm
looking for something which works like this:

--8<-snip->8--

Tag tag1 = new Tag( "java" );
Tag tag2 = new Tag( "means" );
Tag tag3 = new Tag( "coffee" );

// no problem
pm.makePersistent( tag1 );

// no problem
pm.makePersistent( tag2 );

// throws SQLException DUPLICATE KEY but should just attach/update
tag3 to database instance
pm.makePersistent( tag3 );

--8<-snip->8--

Thanks for any hint!

On 20 Dez., 16:02, Dyreatn....TakeThisOut@sun.com wrote:

> But presumably the 'name' column is declared 'UNIQUE' for a reason?
> Allowing you to just add tag3 would violate the unique-ness,
> wouldn't it? If you don't really need them to be unique after all,
> shouldn't you change the table definition to reflect that?

To simplify matters I didn't explain the whole situation but I guess
this is necessary now:

Let's say we have another class, for example a "BlogEntry" class,
which contains - among other things - a Set of Tag objects. Since
there can be many BlogEntry and Tag instances I want the tags to be
unique in the database scheme. blog entries and tags are related to
each other via a join table which contains the columns "blog_id" and
"tag_id".

What I was hoping for is that JDO automatically handles the uniqueness
of tags when persisting a BlogEntry object with n Tag objects and
inserts the right ids into the join table.