Retrieving documents w/ promises - Q.ninvoke

gistfile1.js

var Q = require('q');

var documents = {
    'a': "Document A",
    'b': "Document B",
    'c': "Document C"
};

/**
 * get a document by key
 *
 * Simulates Mongoose Model method
 * @param  {String} key in document
 * @callback {Function}
 * @return {void}
 */
var model = function () {};
model.prototype.getDocument = function (key, done) {
    setTimeout(function () {
        if(documents[key]) {
            return done(null, documents[key]);
        }

        done(new Error('Invalid document'));
    }, 0);
};

var getDocumentWrapper = function (key) {
    var modelInstance = new model();
    return Q.ninvoke(modelInstance, 'getDocument', key);
};

getDocumentWrapper('a')
    .then(function (result) {
        console.log(result);

        return getDocumentWrapper('d');
    })
    .fail(function (err) {
        console.log('Error thrown =(');
        console.error(err);
    });

Output from running is:

Document A
Error thrown =(
[Error: Invalid document]