One of the major issues when dealing with data marshaling is validation. When users input information to your application, you'll often have to validate that information before passing it on to MongoDB. While you can validate your data at the logic layer of your application, it is more useful to do it at the model level. Luckily, Mongoose supports both simple predefined validators and more complex custom validators. Validators are defined at the field level of a document and are executed when the document is being saved. If a validation error occurs, the save operation is aborted and the error is passed to the callback.
Mongoose supports different types of predefined validators, most of which are type-specific. The basic validation of any application is of course the existence of value. To validate field existence in Mongoose, you'll need to use the required property in the field you want to validate. Let's say you want to verify the existence of a username...