Queue - May/June 2008

a handler class. Similarly, GORM has rules for defining which classes to persist and how to include defaults for column and table names. Because of CoC, a typical Grails application contains significantly less configuration code and metadata than an application using a traditional framework.

Now that we have looked at the key underpinnings of GORM, let’s learn how to use it.

GORM MAPPING

A key part of using an ORM framework is specifying how the object model maps to the database. The developer must specify how classes map to tables, attributes map to columns, and relationships map to either foreign keys or join tables. This section looks at how this works using a traditional ORM framework and then how it’s accomplished in Grails.

MAPPING WITH XML AND ANNOTATIONS

The persistent state of a Java class is either its fields or its properties. A field is the Java equivalent of an instance variable. A property is defined by getter and setter methods that follow the JavaBeans¹⁰ naming conventions. For example, getFoo() and setFoo() define the property called foo. The getter and setter methods often provide access to a field of the same name as the property, although they are not required to do so.

A Hibernate application can map the fields or properties of domain classes to the database schema using either XML or annotations. Figure 2 shows an annotation example on the left and an XML example on the right. Both examples persist the fields of the Cus- tomer class, but an applica- tion can persist properties either by annotating the private String name; getter methods or by omitting the default-access attribute from the XML document.

XML and annotations produce equivalent metadata. They both specify that the Customer class is persistent. They also specify that Hibernate

should generate an object’s primary key using whatever mechanism is appropriate for the underlying database and store it in the id field. The version field is configured to store a Hibernate-maintained version number. They both persist the name field and specify that the accounts field represents a one-to-many relationship.

XML and annotations both have defaults for table and column names. The table name defaults to the name of the class and the column name defaults to the name of the property. You can override these defaults using extra annotations or XML attributes and elements. For example, you can specify the table name using the @Table annotation or the name attribute of the <class> element.

Each approach has benefits and drawbacks. One advantage that XML has over annotations is that it separates the O/R mapping from the Java code, which decouples the domain classes from Hibernate. One problem with this separation is that it can be more difficult to keep the mapping and code in sync. XML also tends to be more verbose than annotations. Moreover, the XML mapping must explicitly list all of the persistent properties of a class, whereas fields of certain basic types such as Customer.name are automatically persistent when using annotations.