title: Mongoose findOne embedded document by _id
body: <p>I am trying to push menus to the embedded document. But I am getting not defined findOne in restaurant. I just want to push some documents into the restaurant's menu categories. As you can see in the schema:</p>
  
  <pre><code>var RestaurantSchema = new mongoose.Schema({
      contactTelphone : String,
      address : String,
      branchID : String,
      email : String,
      restaurantName : String,
      userID : String,
      menuCategory : [MenuCategorySchema]
  });
  
  var MenuCategorySchema = new mongoose.Schema({
      menuCategory : String,
      menuCategoryAlt : String,
      sequence : Number,
      menus : [MenuSchema],
      restaurantInfo : { type: Schema.Types.ObjectId, ref: 'Restaurant' },
  });
  
  var MenuSchema = new mongoose.Schema({
      foodName : String,
      foodNameAlt : String,
      picName : String,
      price : String,
      rating : Number,
      menuSequence : Number,
      category : { type: Schema.Types.ObjectId, ref: 'MenuCategory' },
  });
  
  exports.menuForMenuCategory = function(newData, callback)
  {
      console.log(newData);
  
      var menuCategoryId = newData.menuCategoryId;
      var restaurantId = newData.restaurantId;
  
      var newMenu = new Menu({
          foodName : newData.foodName,
          foodNameAlt : newData.foodNameAlt,
          picName : newData.picName,
          price : newData.price,
          cookingCategory : newCookingCategory,
          dishSpecial : newDishSpeical
      });
  
      Restaurant.findOne( {'_id' : restaurantId }, function(err, restaurant){
          if (!err) {
                      //Is it how to do this? It says "findOne not defined"
              restaurant.findOne( "_id" : menuCategoryId, function(err, category){
                  category.push(newMenu);
              });
          }
      });
  }
  </code></pre>