bidirectional one-to-many and unidirectional one-to-many are confused as many-to-many

class Group {

    // Every user has a bunch of groups that it owns
    static belongsTo = [ owner : User ]

    // In addition, every group has members that are users
    static hasMany = [ members : User ]
}


class User {
           
    // Every user has a bunch of groups that it owns
    static hasMany = [ groups: Group ]
    static mappedBy = [ groups:"owner" ]
}

Running script /opt/local/grails-1.0.1/scripts/TestApp.groovy
[0] spring.GrailsWebApplicationContext Refreshing org.codehaus.groovy.grails.commons.spring.GrailsWebApplicationContext@299c3e: display name [org.codehaus.groovy.grails.commons.spring.GrailsWebApplicationContext@299c3e]; startup date [Wed Mar 19 11:27:26 PDT 2008]; root of context hierarchy
[0] spring.GrailsWebApplicationContext Bean factory for application context [org.codehaus.groovy.grails.commons.spring.GrailsWebApplicationContext@299c3e]: org.springframework.beans.factory.support.DefaultListableBeanFactory@4eba57
org.codehaus.groovy.grails.exceptions.GrailsDomainException: No owner defined between domain classes [class User] and [class Group] in a many-to-many relationship. Example: def belongsTo = Group
	at Package_groovy$_run_closure5_closure22.doCall(Package_groovy:231)
	at Package_groovy$_run_closure5_closure22.doCall(Package_groovy)
	at Init_groovy$_run_closure6.doCall(Init_groovy:127)
	at Package_groovy$_run_closure5.doCall(Package_groovy:221)
	at Package_groovy$_run_closure5.doCall(Package_groovy)
	at Bootstrap_groovy$_run_closure2.doCall(Bootstrap_groovy:56)
	at Bootstrap_groovy$_run_closure2.doCall(Bootstrap_groovy)
	at TestApp_groovy$_run_closure9.doCall(TestApp_groovy:259)
	at TestApp_groovy$_run_closure9.doCall(TestApp_groovy)
	at TestApp_groovy$_run_closure3.doCall(TestApp_groovy:115)
	at TestApp_groovy$_run_closure3.doCall(TestApp_groovy)
	at TestApp_groovy$_run_closure1.doCall(TestApp_groovy:63)
	at TestApp_groovy$_run_closure1.doCall(TestApp_groovy)
	at gant.Gant.dispatch(Gant.groovy:271)
	at gant.Gant.this$2$dispatch(Gant.groovy)
	at gant.Gant.invokeMethod(Gant.groovy)
	at gant.Gant.processTargets(Gant.groovy:436)
	at gant.Gant.processArgs(Gant.groovy:372)
	at org.codehaus.groovy.tools.GroovyStarter.rootLoader(GroovyStarter.java:101)
	at org.codehaus.groovy.tools.GroovyStarter.main(GroovyStarter.java:130)
Error loading plugin manager: No owner defined between domain classes [class User] and [class Group] in a many-to-many relationship. Example: def belongsTo = Group