## 5.2 Domain Modelling in GORM

When building Grails applications you have to consider the problem domain you are trying to solve. For example if you were building an Amazon bookstore you would be thinking about books, authors, customers and publishers to name a few.

These are modeled in GORM as Groovy classes so a `Book` class may have a title, a release date, an ISBN number and so on. The next few sections show how to model the domain in GORM.

To create a domain class you can run the create-domain-class target as follows:

    grails create-domain-class Book

The result will be a class at `grails-app/domain/Book.groovy`:

    class Book {
    }

> If you wish to use packages you can move the Book.groovy class into a sub directory under the domain directory and add the appropriate `package` declaration as per Groovy (and Java's) packaging rules.

The above class will map automatically to a table in the database called `book` (the same name as the class). This behaviour is customizable through the ORM Domain Specific Language

Now that you have a domain class you can define its properties as Java types. For example:

    class Book {
    	String title
    	Date releaseDate
    	String ISBN
    }

Each property is mapped to a column in the database, where the convention for column names is all lower case separated by underscores. For example `releaseDate` maps onto a column `release_date`. The SQL types are auto-detected from the Java types, but can be customized via Constraints or the ORM DSL.