How to display a Mongo field on Nodejs with Mongoose and Jade?

Currently everything I see are displaying an entire document, not just one field. How would I display just one field?

I have tried the following but I'm getting "undefined"s where I don't see anything displayed.

Way 1 main js:

// app.js

var schema = new mongoose.Schema({ a_field : String }, 
                                 { collection : 'the_col_with_data' });
var DocumentModel = mongoose.model('Doc', schema);
var a_document = DocumentModel.findOne({}, callback);

app.get('/', function(req, res) {
    res.render('index', {
                            pass_var: a_document.a_field
                        });
});

Way 1 view:

// index.jade

#{ pass_var }

Way 2 main js:

// app.js

var schema = new mongoose.Schema({ a_field : String }, 
                                 { collection : 'the_col_with_data' });
var DocumentModel = mongoose.model('Doc', schema);
var a_document = DocumentModel.findOne({}, callback);

app.get('/', function(req, res) {
    res.render('index', {
                            pass_var: a_document
                        });
});

Way 2 view:

// index.jade

#{ pass_var.a_field }

Interaction with mongoose is asynchronous. That means, you can't rely on the return value of a mongoose operation. You have to perform the logic that operates on your document within the callbacks. So in your case, a_document doesn't point to the document you're trying to find. Instead, you have to use the document within your callback:

app.get('/', function(req, res) {
    DocumentModel.findOne({}, function(err, doc) {
        if(err) {
            res.send(500);
            return;
        }
        res.render('index', {pass_var: doc});
    });
});