How to save Father and Child document with MongooseJS - mongodb

Question

I'm quite new in MongoDB & MongooseJS. Actually learning the MEAN stack, I already love it.
I'm not sure to exactly understand noSQL principles, sorry If I duplicate.
I want to join 2 documents, type father and child, with 1 to N relationship.
Here's two solutions:
• Sub documents: http://mongoosejs.com/docs/subdocs.html
• References: http://mongoosejs.com/docs/populate.html
I've chosen the first one, simpler and IHMO closer to my problem.
Here's the code:
var personSchema = new Schema({
name : String
});
var groupSchema = new Schema({
name : String,
persons : [ personSchema ]
});
var Group = module.exports = mongoose.model('Group', groupSchema);
var Person = module.exports = mongoose.model('Person', personSchema);
Group.findById(groupId, function(err, group) {
var person = new Person();
person.name = "john";
group.persons.push(person);
group.save();
});
When I check for all groups, it work perfectly. The groups are returned, with the people saved.
Group.find(function(err, group) {
// group is full of groups & people
}
But when I check for person, nothing is returned.
Person.find(function(err, person) {
// person is empty
}
It seems that only the Group table document was filled. Is the solution implies to save 1/ the person and 2/ the group, and if so, will Person be saved in two different places (in Group document and in Person document)?
Group.findById(groupId, function(err, group) {
var person = new Person();
person.name = "john";
group.persons.push(person);
person.save(function(...) {
group.save();
});
});
Thanks