class Department(Document):
	name = StringField()

class User(Document):
	name = StringField()
	department_ref = ListField(ReferenceField(Department), default=list)

user1 = User(name='user1').save()
user2 = User(name='user2').save()
user3 = User(name='boss').save()

department1 = Department(name='department1').save()
department2 = Department(name='department2').save()
department3 = Department(name='department3').save()

user1.department_ref = (department1, department2)
user1.save()
user2.department_ref = (department2, department3)
user2.save()
user3.department_ref = (department1, department3)

--
I want to have users with departments like:

'department1', 'user1'
'department2', 'user1'
'department2', 'user2'
'department3', 'user2'

unnest departments to order by departments, users