grails Objects query when hasMany relationship is NULL - mongodb

Question

Not able to get the list of Objects when a hasMany attribute is null.
Class User {
...
List<EMail> emails
static hasMany = [emails: EMail,... ]
static mappedBy = [emails: 'customer',...]
...
}
Where Email is another Class with some String Attributes
Now i am trying to make a simple query as:
Method 1:
def users = User.findAllByEmailsIsEmpty()
This is giving Error as:
Queries of type IsEmpty are not supported by this implementation
Method 2:
def users = User.findAllByEmailsIsNull()
This is giving all the users even those have Email object associated with it.
Then I thought of trying Criteria Query (https://grails.github.io/grails-doc/latest/ref/Domain%20Classes/createCriteria.html )
Method 3:
def userCriteria = User.createCriteria()
def users = userCriteria.list(){
sizeEq('emails', 0)
}
This gives No result ( users.size() is 0 )
Method 4:
def userCriteria = User.createCriteria()
def users = userCriteria.list(){
isNull('emails')
}
This again gives all the Users even those who don't have emails.
Method 5:
def userCriteria = User.createCriteria()
def users = userCriteria.list(){
isEmpty('emails')
}
This gives the Error :
Queries of type IsEmpty are not supported by this implementation
Method 6:
def userCriteria = User.createCriteria()
def users = userCriteria.list(){
eq('emails', null)
}
This again lists down all the users.
PS: Grails is configured with Database as MongoDB.

Answer 1

I would use the grep method. Something like this:
nullEmailUsers = User.list().grep {
!it.emails
}

Answer 2

User.findAll { emails.id != null } do the job!