def book = Book.findByTitle("Groovy in Action")
book
.addToAuthors(name:"Dierk Koenig")
.addToAuthors(name:"Guillaume LaForge")
.save()6 Object Relational Mapping (GORM)
Version: 6.1.0
6 Object Relational Mapping (GORM)
Domain classes are core to any business application. They hold state about business processes and hopefully also implement behavior. They are linked together through relationships; one-to-one, one-to-many, or many-to-many.
GORM is Grails' object relational mapping (ORM) implementation. Under the hood it uses Hibernate (a very popular and flexible open source ORM solution) and thanks to the dynamic nature of Groovy with its static and dynamic typing, along with the convention of Grails, there is far less configuration involved in creating Grails domain classes.
You can also write Grails domain classes in Java. See the section on Hibernate Integration for how to write domain classes in Java but still use dynamic persistent methods. Below is a preview of GORM in action:
6.1 Quick Start Guide
A domain class can be created with the create-domain-class command:
grails create-domain-class myapp.Person| If no package is specified with the create-domain-class script, Grails automatically uses the application name as the package name. |
This will create a class at the location grails-app/domain/myapp/Person.groovy such as the one below:
package myapp
class Person {
}
If you have the dbCreate property set to "update", "create" or "create-drop" on your DataSource, Grails will automatically generate/modify the database tables for you.
|
You can customize the class by adding properties:
class Person {
String name
Integer age
Date lastVisit
}grails consoleThis loads an interactive GUI where you can run Groovy commands with access to the Spring ApplicationContext, GORM, etc.
6.1.1 Basic CRUD
Try performing some basic CRUD (Create/Read/Update/Delete) operations.
Create
To create a domain class use Map constructor to set its properties and call save:
def p = new Person(name: "Fred", age: 40, lastVisit: new Date())
p.save()The save method will persist your class to the database using the underlying Hibernate ORM layer.
Read
Grails transparently adds an implicit id property to your domain class which you can use for retrieval:
def p = Person.get(1)
assert 1 == p.idThis uses the get method that expects a database identifier to read the Person object back from the database.
You can also load an object in a read-only state by using the read method:
def p = Person.read(1)In this case the underlying Hibernate engine will not do any dirty checking and the object will not be persisted. Note that if you explicitly call the save method then the object is placed back into a read-write state.
In addition, you can also load a proxy for an instance by using the load method:
def p = Person.load(1)This incurs no database access until a method other than getId() is called. Hibernate then initializes the proxied instance, or throws an exception if no record is found for the specified id.
Update
To update an instance, change some properties and then call save again:
def p = Person.get(1)
p.name = "Bob"
p.save()6.2 Further Reading on GORM
For more information on using GORM see the dedicated documentation for the GORM project.