According to the Department of Dino Zoo Control (DDZC), the zoo isn’t safe with more than 10 of any one type of Dinosaur.

At this point your Dinosaur model has name, count, and risk properties and you have some server tests written. One of those tests expects a warning message when adding more than 10 dinosaurs, but you receive the following error message:

This error occurred because the Dinosaur model has no validation! You can ensure the safety of the zoo by adding a validator function.

In this exercise you’ll be using a custom validator function. It receives the value to validate as its first argument. It returns a Boolean, which is false if the value fails validation. Avoid arrow notation () =>. Using function() notation preserves the proper binding of this. Here’s the syntax:

// Define validator
validate = function (value) {
  ...
}

// Add validator to Schema
const DinosaurSchema = new Schema({
  count: {
    type: Number,
    validate: [validator, 'custom err msg']
  }
});

Since validation is a model-level concern, you’ll need to test at the model layer. You can test validation like this:

1. Create an instance of a model with validators and execute the validations with the validateSync method. Any validation errors will be stored in [instance].errors.[path], like dino.errors.count.
2. Make assertions on [instance].errors.[path] and its properties.

For more information on validators visit the Validation section of the Mongoose guide: http://mongoosejs.com/docs/validation.html.