Hi,
This is a newbie question, any help would be appreciated. I've read over the quickstart guide, FAQ, chapter 2 of the book, etc, and I must be missing something simple because I just can't get this association to work. The complete details are below: basically the classes and Hibernate access to them is working fine individually, but when I try to associate one to the other it a fails.

I have two database tables, Car and Trim, like so:
CREATE TABLE `cars` (
`id` int(10) unsigned NOT NULL auto_increment,
`make_id` int(10) unsigned NOT NULL default '0',
`model_id` int(10) unsigned NOT NULL default '0',
`year` int(10) unsigned NOT NULL default '0' COMMENT 'e.g. 1996'
)

CREATE TABLE `trims` (
`id` int(10) unsigned NOT NULL auto_increment,
`car_id` int(10) unsigned NOT NULL default '0',
`name` text NOT NULL,
`description` text NOT NULL COMMENT,
PRIMARY KEY (`id`),
KEY `FK_trims_1` (`car_id`),
CONSTRAINT `FK_trims_1` FOREIGN KEY (`car_id`) REFERENCES `cars` (`id`)
)

I have simple Java POJO classes for these, com.mycompany.Car and com.mycompany.Trim, following the JavaBean conventions as shown in the Hibernate reference documentation. I have Managers and unit tests for these classes that all work fine. The complete mapping files are below.

I'd like a unidirectional association from Car to Trims, i.e. one car has many trims. If I understand the Hibernate terminology correctly, Car is the parent, Trim is a child. This is a Set association, as there is some number of trims and the order does not matter.

So I added the following methods to thr Car class:
private Set trims = new HashSet();
public Set getTrims() { return trims; }
public void setTrims(Set trims) { this.trims = trims; }

*** This is where I need help: ***
What do I add to the Car.hbm.xml mapping files to make this work? I tried this and got the error shown below:
<set name="trims" inverse="true">
<key column="car_id"/>
<one-to-many class="com.mycompany.Trim"/>
</set>

Hibernate version: 3.1.2

Mapping documents:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
<class name="com.mycompany.Car" table="CARS">
<id name="id" column="ID">
<generator class="native" />
</id>
<property name="makeId" column="MAKE_ID" />
<property name="modelId" column="MODEL_ID" />
<property name="year" />
</class>
</hibernate-mapping>

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
<class name="com.mycompany.Trim" table="TRIMS">
<id name="id" column="ID">
<generator class="native" />
</id>
<property name="carId" column="CAR_ID" />
<property name="name" />
<property name="description" />
</class>
</hibernate-mapping>

Full stack trace of any exception that occurs:
Mar 14, 2006 1:39:01 PM org.hibernate.LazyInitializationException <init>
SEVERE: failed to lazily initialize a collection of role: com.nimalex.domain.Car
.trims, no session or session was closed
org.hibernate.LazyInitializationException: failed to lazily initialize a collect
ion of role: com.nimalex.domain.Car.trims, no session or session was closed
at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitia
lizationException(AbstractPersistentCollection.java:358)
at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitia
lizationExceptionIfNotConnected(AbstractPersistentCollection.java:350)
at org.hibernate.collection.AbstractPersistentCollection.initialize(Abst
ractPersistentCollection.java:343)
at org.hibernate.collection.AbstractPersistentCollection.read(AbstractPe
rsistentCollection.java:86)
at org.hibernate.collection.PersistentSet.toString(PersistentSet.java:25
3)
at java.lang.String.valueOf(String.java:2577)
at java.lang.StringBuilder.append(StringBuilder.java:116)
at com.nimalex.domain.Car.toString(Car.java:147)
at java.lang.String.valueOf(String.java:2577)
at java.lang.StringBuilder.append(StringBuilder.java:116)
at com.nimalex.domain.CarManagerTest.testGet(CarManagerTest.java:115)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.
java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAcces
sorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at junit.framework.TestCase.runTest(TestCase.java:154)
at junit.framework.TestCase.runBare(TestCase.java:127)
at junit.framework.TestResult$1.protect(TestResult.java:106)
at junit.framework.TestResult.runProtected(TestResult.java:124)
at junit.framework.TestResult.run(TestResult.java:109)
at junit.framework.TestCase.run(TestCase.java:118)
at junit.framework.TestSuite.runTest(TestSuite.java:208)
at junit.framework.TestSuite.run(TestSuite.java:203)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.
java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAcces
sorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.apache.maven.surefire.battery.JUnitBattery.executeJUnit(JUnitBatt
ery.java:242)
at org.apache.maven.surefire.battery.JUnitBattery.execute(JUnitBattery.j
ava:216)
at org.apache.maven.surefire.Surefire.executeBattery(Surefire.java:215)
at org.apache.maven.surefire.Surefire.run(Surefire.java:163)
at org.apache.maven.surefire.Surefire.run(Surefire.java:87)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.
java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAcces
sorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.apache.maven.surefire.SurefireBooter.runTestsInProcess(SurefireBo
oter.java:285)
at org.apache.maven.surefire.SurefireBooter.run(SurefireBooter.java:201)

at org.apache.maven.test.SurefirePlugin.execute(SurefirePlugin.java:366)

at org.apache.maven.plugin.DefaultPluginManager.executeMojo(DefaultPlugi
nManager.java:415)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoals(Defa
ultLifecycleExecutor.java:531)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalWithLi
fecycle(DefaultLifecycleExecutor.java:472)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoal(Defau
ltLifecycleExecutor.java:451)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalAndHan
dleFailures(DefaultLifecycleExecutor.java:303)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeTaskSegmen
ts(DefaultLifecycleExecutor.java:270)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.execute(DefaultLi
fecycleExecutor.java:139)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:322)
at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:115)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:249)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.
java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAcces
sorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.codehaus.classworlds.Launcher.launchEnhanced(Launcher.java:315)
at org.codehaus.classworlds.Launcher.launch(Launcher.java:255)
at org.codehaus.classworlds.Launcher.mainWithExitCode(Launcher.java:430)

at org.codehaus.classworlds.Launcher.main(Launcher.java:375)
Mar 14, 2006 1:39:01 PM com.nimalex.domain.CarManagerTest testGet
SEVERE: Failed test
org.hibernate.LazyInitializationException: failed to lazily initialize a collect
ion of role: com.nimalex.domain.Car.trims, no session or session was closed
at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitia
lizationException(AbstractPersistentCollection.java:358)
at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitia
lizationExceptionIfNotConnected(AbstractPersistentCollection.java:350)
at org.hibernate.collection.AbstractPersistentCollection.initialize(Abst
ractPersistentCollection.java:343)
at org.hibernate.collection.AbstractPersistentCollection.read(AbstractPe
rsistentCollection.java:86)
at org.hibernate.collection.PersistentSet.toString(PersistentSet.java:25
3)
at java.lang.String.valueOf(String.java:2577)
at java.lang.StringBuilder.append(StringBuilder.java:116)
at com.nimalex.domain.Car.toString(Car.java:147)
at java.lang.String.valueOf(String.java:2577)
at java.lang.StringBuilder.append(StringBuilder.java:116)
at com.nimalex.domain.CarManagerTest.testGet(CarManagerTest.java:115)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.
java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAcces
sorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at junit.framework.TestCase.runTest(TestCase.java:154)
at junit.framework.TestCase.runBare(TestCase.java:127)
at junit.framework.TestResult$1.protect(TestResult.java:106)
at junit.framework.TestResult.runProtected(TestResult.java:124)
at junit.framework.TestResult.run(TestResult.java:109)
at junit.framework.TestCase.run(TestCase.java:118)
at junit.framework.TestSuite.runTest(TestSuite.java:208)
at junit.framework.TestSuite.run(TestSuite.java:203)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.
java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAcces
sorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.apache.maven.surefire.battery.JUnitBattery.executeJUnit(JUnitBatt
ery.java:242)
at org.apache.maven.surefire.battery.JUnitBattery.execute(JUnitBattery.j
ava:216)
at org.apache.maven.surefire.Surefire.executeBattery(Surefire.java:215)
at org.apache.maven.surefire.Surefire.run(Surefire.java:163)
at org.apache.maven.surefire.Surefire.run(Surefire.java:87)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.
java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAcces
sorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.apache.maven.surefire.SurefireBooter.runTestsInProcess(SurefireBo
oter.java:285)
at org.apache.maven.surefire.SurefireBooter.run(SurefireBooter.java:201)

at org.apache.maven.test.SurefirePlugin.execute(SurefirePlugin.java:366)

at org.apache.maven.plugin.DefaultPluginManager.executeMojo(DefaultPlugi
nManager.java:415)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoals(Defa
ultLifecycleExecutor.java:531)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalWithLi
fecycle(DefaultLifecycleExecutor.java:472)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoal(Defau
ltLifecycleExecutor.java:451)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeGoalAndHan
dleFailures(DefaultLifecycleExecutor.java:303)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.executeTaskSegmen
ts(DefaultLifecycleExecutor.java:270)
at org.apache.maven.lifecycle.DefaultLifecycleExecutor.execute(DefaultLi
fecycleExecutor.java:139)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:322)
at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:115)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:249)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.
java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAcces
sorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.codehaus.classworlds.Launcher.launchEnhanced(Launcher.java:315)
at org.codehaus.classworlds.Launcher.launch(Launcher.java:255)
at org.codehaus.classworlds.Launcher.mainWithExitCode(Launcher.java:430)

at org.codehaus.classworlds.Launcher.main(Launcher.java:375)

Name and version of the database you are using:
MySQL 5.0.18-nt

The generated SQL (show_sql=true):
Hibernate: select car0_.ID as ID0_, car0_.MAKE_ID as MAKE2_0_, car0_.MODEL_ID as
MODEL3_0_, car0_.year as year0_ from CARS car0_ where car0_.MAKE_ID=2 and car0_
.MODEL_ID=2 and car0_.year=1964

Again, any help would be appreciated. Thank you in advance,

Yoav Shapira

http://www.hibernate.org/162.html

set lazy="false" for the collection and/or class and you should get past this error, then read up on that to decide if you want to load your collection lazily or not, can be either way.

Added lazy="false" to the <set> element solved the problem. Thank you for your help.