How to save populated Document?

How to save populated Document?

Is it possible to save a populated document?
I am trying to do:
var Group = new Db['Group']();
for (var i=0; i<50; i++)
Db.Members.push({ User: { _id: "521014731e27b1b008000002"}, pseudo:
'John' });
Group.save();
Schemas
var GroupSchemaModel = {
Members: [{
User: { type: mongoose.Schema.Types.ObjectId, ref: 'User' },
updated_at: { type: Date, required: true, default: Date.now }
}]
};
I get the error
{ message: 'Cast to ObjectId failed for value "[object Object]" at path
"User"',
name: 'CastError',
type: 'ObjectId',
value: { _id: '521014731e27b1b008000002' },
path: 'User' }