Bar.groovy

package foo

class Bar {

  String name
}

Foo.groovy

package foo

class Foo {

  Bar bar

  static constraints = {
    bar nullable: true
  }
}

FooIntTest.groovy

package foo

class FooIntTest extends GroovyTestCase {

  FooService fooService

  void testList() {
    Bar bar = new Bar(name: 'first').save()
    Bar bar2 = new Bar(name: 'second').save()

    Foo foo = new Foo(bar: bar).save()
    Foo foo2 = new Foo(bar: bar2).save()
    Foo foo3 = new Foo(bar: null).save()

    List<Foo> foos = fooService.list()

    assert foos.size() == 1
  }
}

FooService.groovy

package foo

class FooService {

  List<Foo> list() {
    Foo.where {
      bar?.name == 'first'
    }.list()
  }
}

stacktrace.log

| Failure:  testList(foo.FooIntTest)
|  org.hibernate.QueryException: could not resolve property: name of: foo.Foo
	at grails.gorm.DetachedCriteria$_list_closure3.doCall(DetachedCriteria.groovy:847)
	at grails.gorm.DetachedCriteria$_withPopulatedQuery_closure10.doCall(DetachedCriteria.groovy:1213)
	at org.grails.datastore.gorm.GormStaticApi$_withDatastoreSession_closure23.doCall(GormStaticApi.groovy:700)
	at org.grails.datastore.mapping.core.DatastoreUtils.execute(DatastoreUtils.java:302)
	at org.grails.datastore.gorm.AbstractDatastoreApi.execute(AbstractDatastoreApi.groovy:37)
	at org.grails.datastore.gorm.GormStaticApi.withDatastoreSession(GormStaticApi.groovy:699)
	at grails.gorm.DetachedCriteria.withPopulatedQuery(DetachedCriteria.groovy:1184)
	at grails.gorm.DetachedCriteria.list(DetachedCriteria.groovy:843)
	at grails.gorm.DetachedCriteria.list(DetachedCriteria.groovy:842)
	at foo.FooService.list(FooService.groovy:6)
	at foo.FooIntTest.testList(FooIntTest.groovy:15)
	at junit.framework.TestCase.runTest(TestCase.java:176)
	at junit.framework.TestCase.runBare(TestCase.java:141)
	at junit.framework.TestResult$1.protect(TestResult.java:122)
	at junit.framework.TestResult.runProtected(TestResult.java:142)
	at junit.framework.TestResult.run(TestResult.java:125)
	at junit.framework.TestCase.run(TestCase.java:129)
	at junit.framework.TestSuite.runTest(TestSuite.java:255)
	at junit.framework.TestSuite.run(TestSuite.java:250)