Mongoose update embedded document does not work

Greeting all,

I defined a deeply embedded mongoose model "Person", which contains a field "Contact", and "Contact" has an array of "Address" for this person's work, home, delivery etc addresses.

When I wanted to update an address, and use the model's "save" function, the change wasn't reflected in the database.

            console.log(person.Contact.Address[i].City = 'Chicago');
            person.save(function (err) {
                if (!err) {
                   console.log(person.Contact.Address[i].City);

The console.log indicates that the field City has been updated to new value, but the mongodb's value stays the same.

Could you please help me with this issue?

Thanks! Gary