def book = Book.findByTitle("Groovy in Action")
book
.addToAuthors(name:"Dierk Koenig")
.addToAuthors(name:"Guillaume LaForge")
.save()7 Object Relational Mapping (GORM)
Version: 7.0.12
7 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:
7.1 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.
7.1.1 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.
List
To retrieve multiple instances, use the list method:
def people = Person.list()This returns all Person records from the database.
You can also pass pagination and sorting parameters:
def fetchParams = [sort: 'name', order: 'asc', max: 10, offset: 0]
def people = Person.list(fetchParams)The list method supports:
-
max– maximum number of results -
offset– starting position -
sort– property to sort by -
order– sort direction (ascordesc)
In addition to these you can specify advanced parameters such as cache, fetch, lock, readOnly, fetchSize, timeout, flushMode, and ignoreCase.
See list for the complete list of supported parameters.
Read
Grails transparently adds an implicit id property to your domain class which you can use for retrieval:
def p = Person.get(1)
assert p.id == 1This 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()You can also update multiple properties at once using the properties assignment:
def p = Person.get(1)
p.properties = [name: "Bob", age: 45]
p.save()This will bind the given map to the domain instance, updating all matching properties in a single step.
Only properties defined in the domain class will be assigned, and any missing properties in the map will remain unchanged.
Delete
To delete an instance use the delete method:
def p = Person.get(1)
p.delete()If a delete operation fails (for example due to database constraints), an exception is thrown.
You can handle this using a try/catch block:
def p = Person.get(1)
try {
p.delete(flush: true)
} catch (Exception e) {
println "Delete failed: ${e.message}"
}Unlike the save method, the delete method does not support a failOnError parameter. Instead, errors are propagated as exceptions.
Using flush: true ensures the delete is executed immediately, so any errors are raised at that point.
Querying
To dynamically build queries based on optional parameters a common pattern is to use DetachedCriteria and progressively compose filters depending on the provided inputs.
Properties
Consider the following example using the Person domain class:
import grails.gorm.DetachedCriteria
private DetachedCriteria<Person> buildQuery(Map filterParams) {
def query = Person.where {}
if (filterParams.containsKey('id')) {
query = query.where { id == filterParams.id }
}
if (filterParams.containsKey('name')) {
query = query.where { name == filterParams.name }
}
if (filterParams.containsKey('age')) {
query = query.where { age == filterParams.age }
}
return query
}Associations
You can filter by associated properties using dot notation or nested criteria. For example, if Person has a self-referencing relationship parent, you can filter by properties of the parent.
class Person {
String name
Integer age
Date lastVisit
Person parent
}Using dot notation:
if (filterParams.containsKey('parent.name')) {
query = query.where { parent.name == filterParams.'parent.name' }
}Using a nested criteria block, which is useful when filtering multiple properties of the association:
if (filterParams.containsKey('parent.name')) {
query = query.where {
parent {
name == filterParams.'parent.name'
}
}
}This approach allows you to:
-
Build queries incrementally
-
Apply only the filters that are actually provided
-
Keep query logic reusable and centralized
You can then use this query in different ways.
Find a Single Result
def filterParams = [id: 1]
def person = buildQuery(filterParams).get()
Note: DetachedCriteria.get() returns a single result from the criteria query, while Person.get(id) is a static lookup by primary key on the domain class.
|
List Results
def filterParams = [age: 40]
def fetchParams = [sort: 'name', order: 'asc']
def people = buildQuery(filterParams).list(fetchParams)Filters can be combined simply by passing multiple items:
def filterParams = [name: "Fred", age: 40]
def results = buildQuery(filterParams).list()Count Results
def filterParams = [age: 40]
def total = buildQuery(filterParams).count()Each condition is applied only if the corresponding parameter exists, making this pattern highly flexible for search forms and APIs.
Notes
-
Each call to
where {}returns a newDetachedCriteria, allowing safe chaining. -
This pattern avoids large, hardcoded query methods.
-
It works seamlessly with GORM and Hibernate in Grails.
This technique is especially useful in service layers where filtering logic must remain dynamic and maintainable.
7.2 Further Reading on GORM
For more information on using GORM see the dedicated documentation for the GORM project.