Eu gostaria de ligarensureIndex noauthorName, qual é o comando e onde neste código devo colocá-lo?

var mongoose = require('mongoose');

// defines the database schema for this object
var schema = mongoose.Schema({
    projectName : String,
    authorName : String,
    comment : [{
        id : String,                                    
        authorName : String,
        authorEmailAddress : { type : String, index : true }    
    }]
});

// Sets the schema for model
var ProjectModel = mongoose.model('Project', schema);

// Create a project
exports.create = function (projectJSON) {
    var project = new ProjectModel({
        projectName : projectJSON.projectName,
        authorName : projectJSON.authorName,    

        comment : [{
            id : projectJSON.comments.id,                                           
            authorName : projectJSON.comments.authorName,                           
            authorEmailAddress : projectJSON.authorEmailAddress
        });

        project.save(function(err) {
            if (err) {
                console.log(err);
            } else{
                console.log("success");
            }
        });
    });
}