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From: Jean-Paul Le =?iso-8859-1?q?F=E8vre?= <jean-paul.lefevre@cea.fr>
Organization: CEA DAPNIA
To: user@cayenne.apache.org
Subject: Re: Problem concerning Prefetching
Date: Mon, 28 Jan 2008 16:29:35 +0100
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On Monday 28 January 2008, Michael Gentry wrote:
> Hi Jean-Paul,

Hi Michael and Andrus,

>
> It looks like galaxy -> galaxy_component_properties is a to-one
> relationship, which would cause Cayenne to issue multiple selects like
> this since it tries to resolve each fault individually.

It's a one to many : each galaxy has 3 different types of properties.

Here is the code (simplified) implementing my method :

public final List<Galaxy> selectGalaxies(int timestep)=20
{
    StringBuffer buf =3D new StringBuffer("halo.simulation.name =3D '");
    buf.append(getCurrentSimulationName());
    buf.append("' and halo.properties.timestep =3D ");
    buf.append(timestep);

    Expression exp    =3D Expression.fromString(buf.toString());
    SelectQuery query =3D new SelectQuery(Galaxy.class, exp);
    query.addOrdering("number", true);

    //query.addPrefetch("componentProperties");=20

    return (List<Galaxy>) performQuery(query);
}

And here is how I use it :
galaxies =3D handler.selectGalaxies(ts);

// Loop on galaxies, get the disc component and the mass of baryons.
for (Galaxy g : galaxies) {

    GalaxyComponentProperties prop =3D g.getComponentProperties("disc");
    if (prop !=3D null) {
        printer.println(g.getNumber() + ", " +
                        prop.getMassOfBaryons().toString());
    }
}

>
> Given your manual SQL statement:
>
> select galaxies.number, galaxy_component_properties.mass_of_baryons
>       from galaxy_component_properties
>       join galaxies on galaxy_component_properties.galaxy_id =3D galaxies=
=2Eid
>       where some_clause
>       order by galaxies.number;
>
> It looks like you don't actually care about having actual Cayenne
> objects (it looks like you only care about having two of the
> attributes)?  If this is the case, you could use an SQLTemplate and
> raw data rows to only fetch in those two properties.  If this isn't
> the case, let us know and we'll give you other ideas.

In fact you asked the good question. I'm just at the beginning of the=20
development and I don't really know how the application will be used and as=
 a=20
consequence it is hard to figure out what it the best strategy to implement=
=20
the functionalities : I guess that some users are only interested in=20
computing mean values, sums, min, max  in big chunks of data but others wan=
t=20
to retrieve all the attributes of a small amount of specific objects. The=20
challenge is to make both types of users happy. I'm wondering whether it is=
=20
possible or not.
In my experience Cayenne has always been excellent at manipulating small=20
numbers of complex objects but, on the other hand, difficult to use on huge=
=20
dataset.



=2D-=20
___________________________________________________________________

Jean-Paul Le F=E8vre  * Mail : LeFevre@fonteny.org