Creating a Foreign Key relationship in Mongoose

I'm beginning with Mongoose and I want to know how to do this type of configuration:

A recipe has different ingredients.

I have my two models:

Ingredient and Recipe:

var mongoose = require('mongoose');
var Schema = mongoose.Schema;

var IngredientSchema = new Schema({
    name: String
});

module.exports = mongoose.model('Ingredient', IngredientSchema);

var mongoose = require('mongoose');
var Schema = mongoose.Schema;

var RecipeSchema = new Schema({
    name: String
});

module.exports = mongoose.model('Recipe', RecipeSchema);

Check Updated code below, in particular this part: {type: Schema.Types.ObjectId, ref: 'Ingredient'}

var mongoose = require('mongoose');
var Schema = mongoose.Schema;

var IngredientSchema = new Schema({
    name: String
});

module.exports = mongoose.model('Ingredient', IngredientSchema);

var mongoose = require('mongoose');
var Schema = mongoose.Schema;

var RecipeSchema = new Schema({
    name: String,
    ingredients:[
      {type: Schema.Types.ObjectId, ref: 'Ingredient'}
    ]
});

module.exports = mongoose.model('Recipe', RecipeSchema);

To Save:

var r = new Recipe();

r.name = 'Blah';
r.ingredients.push('mongo id of ingredient');

r.save();