This is an overview of Grails Object Relational Mapping aka GORM.

1. Overview of GORM
Chris Richardson
Author of POJOs in Action
www.chrisrichardson.net

2. About Chris
Grew up in England and live in Oakland, CA
Over 20+ years of software development
experience including 12 years of Java
Author of POJOs in Action
Speaker at JavaOne, SpringOne, NFJS,
JavaPolis, Spring Experience, etc.
Chair of the eBIG Java SIG in Oakland
(www.ebig.org)
Run the Groovy/Grails meetup
(http://java.meetup.com/161)
Run a consulting and training company that
u co su t g a d t a g co pa y t at
helps organizations build better software faster
and deploy it on Amazon EC2
Founder of Cloud Tools, an open-source project
for deploying Java applications on Amazon EC2:
http://code.google.com/p/cloudtools
http://code google com/p/cloudtools
Founder of a startup that provides outsourced,
automated, and Java-centric datacenter
management on the cloud:
www.cloudfoundry.com
y
Slide 2
Copyright (c) 2009 Chris Richardson. All rights reserved.

3. What’s GORM?
GORM = Grails Object Relational Mapping
Built
B ilt on Hibe nate
Hibernate
Leverages the power of Hibernate
But is very different - simpler, easier to use API
y p,
Uses convention over configuration
Defaults for which classes to persist
Defaults for their O/R mapping
Leverages the meta-object protocol
Adds persistence methods to domain classes
p
No equivalent of Hibernate Session
Avoids the need for dependency injection
Eliminates many DAO cookie-cutter methods
Slide 3
Copyright (c) 2009 Chris Richardson. All rights reserved.

4. Database access made easy
y
class Customer {
String name
g
}
Customer c = new Customer(quot;John Doequot;)
Ct Ct (quot;J h D quot;)
if (!c.save())
fail quot;validation failed: ${c.errors}quot;
Customer c2 = Customer.get(c.id)
No dependency
injection
c2.delete()
assertNull Customer.get(c.id)
def customers = Customer findAllByName(“Fred”)
Customer.findAllByName( Fred )
Slide 4
Copyright (c) 2009 Chris Richardson. All rights reserved.

5. Relationships don’t have to be
difficult
class Customer {
static hasMany = [ accounts : Account]
}
class Account {
static belongsTo = [customer: Customer]
double balance
}
Customer c = <…>
Account a = new Account(…)
c.addToAccounts(a)
assertSame c, a.customer
assertTrue c.accounts.contains(a)
Slide 5
Copyright (c) 2009 Chris Richardson. All rights reserved.

6. When the defaults aren’t right
g
class Customer {
static transients = [quot;networthquot;]
[ ]
static mapping = {
id column: 'customer_id'
table 'crc_customer'
_
columns {
name column: 'customer_name'
}
}
def getNetworth() { …}
…
}
Slide 6
Copyright (c) 2009 Chris Richardson. All rights reserved.

7. Working with constraints
g
class Customer {
static constraints = {
name(size: 5..15, blank: false)
}
String name
}
Checked by the UI
Checked by save()
Used by GORM to generate schema constraints
Slide 7
Copyright (c) 2009 Chris Richardson. All rights reserved.

8. More on dynamic finders
y
Rich query language
Various methods: fi dB () fi dAllB ()
Vi th d findBy…(), findAllBy…(),
countBy…()
Various operators: e.g
findAllByFirstNameAndLastName(…),
findAllByBalanceLessThan(…),
findAllByStatusIsNotNull()
y ()
BUT:
Limited to single class/table, i.e. no joins
Lack of encapsulation – the name of the
finder reflects the implementation – not the
meaning
Slide 8
Copyright (c) 2009 Chris Richardson. All rights reserved.

9. When dynamic finders are
insufficient
HQL
List accounts = Account.findAll(“from Account where balance < ?”, [threshold])
def c = Account.createCriteria()
def results = c.list {
lt( balance
lt(“balance”, threshold)
}
Slide 9
Copyright (c) 2009 Chris Richardson. All rights reserved.

10. Mapping with XML or annotations
pp g
Using .hbm.xml files
Create grails-
app/conf/hibernate/hibernate.cfg.xml
Define .hbm.xml mapping files
D fi hb l i fil
Map either Groovy or Java classes
Using JPA annotations
Ui t ti
Annotate Java classes
Create hib
C hibernate.cfg.xml
f l
You can still use the dynamically
defined
d fi d GORM methods i G
th d in Groovy
Slide 10
Copyright (c) 2009 Chris Richardson. All rights reserved.

11. Under the covers
Creates DataSource specified in grails-
app/config/DataSource.groovy
Creates Hibernate SessionFactory
Use ExpandoMetaClass mechanism
Add persistence methods, e.g.
domainClass.metaClass.'static'.get = {…}
Adds methodMissing() to handle dynamic
finders
fi d
These methods have a reference to the
SessionFactory
Slide 11
Copyright (c) 2009 Chris Richardson. All rights reserved.

12. GORM is great but …
g
GORM does not support multiple
DataSources
Lack of SOC
Data access logic is scattered around the
application rather than being
encapsulated by DAOs
CoC-based mapping works best for
new databases
Disadvantages of programming in a
dynamic language
Slide 12
Copyright (c) 2009 Chris Richardson. All rights reserved.

13. Further reading
g
My ACM Queue article:
http://queue.acm.org/detail.cfm?id=1394140
Slide 13
Copyright (c) 2009 Chris Richardson. All rights reserved.