Cayenne User Documentation
Tutorial Delete

This section explains how to model relationship delete rules and how to delete individual objects as well as sets of objects. Also demonstrated the use of `DataObjectUtils` class to run a query.

### Setting Up Delete Rules

Before we discuss the API for object deletion, lets go back to CayenneModeler and set up some delete rules. Doing this is optional but will simplify correct handling of the objects related to deleted objects.

In the Modeler go to "Artist" ObjEntity, "Relationships" tab and select "Cascade" for the "paintings" relationship delete rule:

Repeat this step for other relationships:

- For Gallery set "paintings" relationship to be "Nullify", as a painting can exist without being displayed in a gallery.
- For Painting et both relationships rules to "Nullify".

Now save the mapping, and refresh the project in Eclispe.

### Deleting Objects

While deleting objects is possible via SQL, qualifying a delete on one or more IDs, a more common way in Cayenne (or ORM in general) is to get a hold of the object first, and then delete it via the context. Let's use utility class DataObjectUtils to find an artist:

    Expression qualifier = ExpressionFactory.matchExp(Artist.NAME_PROPERTY, "Pablo Picasso");
    SelectQuery select = new SelectQuery(Artist.class, qualifier);
    Artist picasso = (Artist) DataObjectUtils.objectForQuery(context, select);

Now let's delete the artist:

    if (picasso != null) {
        context.deleteObject(picasso);
        context.commitChanges();
    }

Since we set up "Cascade" delete rule for the Artist.paintings relationships, Cayenne will automatically delete all paintings of this artist. So when your run the app you'll see this output:

    INFO: SELECT t0.DATE_OF_BIRTH, t0.ID, t0.NAME FROM ARTIST t0 WHERE t0.NAME = ?
     [bind: 1->NAME:'Pablo Picasso']
    INFO: === returned 1 row. - took 5 ms.
    INFO: +++ transaction committed.
    INFO: --- will run 2 queries.
    INFO: --- transaction started.
    INFO: DELETE FROM PAINTING WHERE ID = ?
    INFO: [batch bind: 1->ID:2]
    INFO: [batch bind: 1->ID:1]
    INFO: === updated 2 rows.
    INFO: DELETE FROM ARTIST WHERE ID = ?
    INFO: [batch bind: 1->ID:1]
    INFO: === updated 1 row.
    INFO: +++ transaction committed.

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