Built-in & Custom Schema Validations with Custom Error Messages

Beginner12 min read•Updated: Jul 11, 2026

Built-in & Custom Schema Validations with Custom Error Messages

Data integrity starts at the model layer. Mongoose provides built-in validators for numbers, strings, dates, and arrays, as well as the ability to define asynchronous Custom Validators that enforce complex business rules before data reaches the database.

1. Built-in Mongoose Validators

Validator	Target Types	Description	Example
`required`	All types	Ensures field exists and is not null	`required: [true, 'Email is mandatory']`
`min` / `max`	Number, Date	Enforces minimum and maximum bounds	`min: [0, 'Price must be positive']`
`minlength` / `maxlength`	String	Restricts string character count	`minlength: [8, 'Password too short']`
`enum`	String	Restricts string to allowed array of values	`enum: ['draft', 'published', 'archived']`
`match`	String	Validates string against a Regular Expression	`match: [/^[\w-\.]+@([\w-]+\.)+[\w-]{2,4}$/, 'Invalid email']`

2. Writing Custom Asynchronous Validators

Custom validators use the validate property. You can supply a synchronous function, an asynchronous function returning a Promise/Boolean, or an async validator with custom error messages:

JavaScript

1import { Schema, model } from 'mongoose';
2 
3const userSchema = new Schema({
name: {
  type: String,
  required: true
},
email: {
  type: String,
  required: true,
  unique: true, // Note: unique creates a database index, not a validator!
  validate: {
    validator: function(v) {
      return /^[\w-\.]+@([\w-]+\.)+[\w-]{2,4}$/.test(v);
    },
    message: props => `${props.value} is not a valid email address!`
  }
},
discountPrice: {
  type: Number,
  validate: {
    validator: function(val) {
      // 'this' points to the current document being created
      return val < this.price;
    },
    message: 'Discount price ({VALUE}) must be less than the regular price'
  }
},
price: {
  type: Number,
  required: true
}
33});

Note: In custom validators, this only points to the document on new document creation (.save()). For updates via findOneAndUpdate(), use { runValidators: true, context: 'query' }.

3. Triggering Validation on Updates

By default, Mongoose executes schema validators when creating documents via .save() or .create(). When running update queries like findByIdAndUpdate() or updateOne(), Mongoose skips validators by default for performance!

To force validation on updates, you must pass { runValidators: true }:

JavaScript

1const updatedCourse = await Course.findByIdAndUpdate(
courseId,
{ price: -50 }, // Invalid!
{
  new: true, // Return updated document
  runValidators: true // Enforces schema validation rules!
}
8);

Interactive Quiz• Knowledge Check

Multiple Choice Questions

Question 1

Why must { runValidators: true } be explicitly specified when calling findByIdAndUpdate() in Mongoose?

Question 2

Is unique: true in a Mongoose schema a validator?

Question 3

In a custom validator function, how do you access the value of the field being validated?

Question 4

Which built-in validator ensures that a String matches a predefined list of allowed choices?

Question 5

What type of error object is thrown when Mongoose validation fails?

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