Working with MongoDB field name with special character

Question

I am a newbie to mongodb. I want to write a query to print part of the column from database.

Here is the data,

{
    "user-id" : "u1",
    "username" : "amy",
},
{
    "user-id" : "u2",
    "username" : "billy",
},
{
    "user-id" : "u3",
    "username" : "charlie",
},
{
    "user-id" : "u4",
    "username" : "david",
}

It's working fine if I print the "username" with

db.user.find().forEach(
    function(myDoc) {
        print( "user: " + myDoc.username );
    });

But I can't print the fields with a hyphen in it,

db.user.find().forEach(
    function(myDoc) {
        print( "user: " + myDoc.user-id );
    });

Error:

Error: Line 3: Unexpected token (

I tried "user-id", "user-id", but its not work. How do I print the fields with a hyphen in it? Thanks.