//Example data of variables
var home = "home1";
var itemId = "123";
var username = Meteor.user().username;

//Upsert command
items.upsert({
    '_id': home
}, {
    $push: {
        'createdItems': {itemId, username, home}

    }

});

"_id" : "home1",
    "createdItems" : [
        {
            "itemId" : "123",
            "username" : "Jon Doe",
            "home" : "home1"
        },
        {
            "itemId" : "456",
            "username" : "Jon Doe2",
            "home" : "home1"
        },
        {
            "itemId" : "789",
            "username" : "Jon Doe3",
            "home" : "home1"
        }
    ]
}

//update a field

    items.upsert({
    'itemId': "123"
    }, {

        $set: {
    'createdItems': {username: "Clark Kent"}
                }

        });

//insert new field

    items.upsert({
    'itemId': "123"
    }, {

        $set: {
    'createdItems': {Value: 1000}
                }

        });

"_id" : "home1",
    "createdItems" : [
        {
            "itemId" : "123",
            "username" : "Clark Kent",
            "home" : "home1",
            "Value" : 1000
        },
        {
            "itemId" : "456",
            "username" : "Jon Doe2",
            "home" : "home1"
        },
        {
            "itemId" : "789",
            "username" : "Jon Doe3",
            "home" : "home1"
        }
    ]
}