addTask

Todo.prototype.addTask = function(date, taskName, callback){
	this.model.findOne({
	  date: date
	}, function(error, data) {
	  var taskId;
	  if (error) {
	    console.error('error:findOne:', error); //optional, so helpful during development. it will tell you exactly where there exception happened.
	    throw new Error(error); //this will make it an instance of the javascript native Error class.
	  }
	  
		if(!data) {
			taskId = 1;
		} else {
			taskId = data.tasks.length + 1;
		}
		
		var newTask = new models.todoModel({ 
		  date : this.date, 
	    tasks : [{ status: false,
  		  taskName: taskName,
  			taskId : taskId
			}]
		});
		
		newTask.save(function(error){
			if(error) {
				throw new Error(error);
			}
			callback(null, true);
		});
	});
};

todoExists

Todo.prototype.todoExists = function (date, callback){
	this.model.findOne({
	  date : date
	}, function(error, data){
		if(error) {
			throw new Error(error);
		}
		
		if (!data) {
		  callback(null, false);
		} else {
		  callback(null, true);
		}
		
	});
	
};