Mongoose Validation and Middleware

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Validation and Hooks in Mongoose

Mongoose runs validation before saving — if a required field is missing or a value falls outside enum, Mongoose throws a ValidationError before the document touches the database.

Middleware (hooks) let you run logic before or after operations like save, validate, and findOneAndUpdate.

import bcrypt from "bcrypt";

const userSchema = new Schema({
  email:    { type: String, required: true, unique: true },
  password: { type: String, required: true, minlength: 8 }
});

// Pre-save hook: hash password before storing
userSchema.pre("save", async function (next) {
  if (!this.isModified("password")) return next();
  this.password = await bcrypt.hash(this.password, 12);
  next();
});

// Instance method
userSchema.methods.comparePassword = async function (candidate) {
  return bcrypt.compare(candidate, this.password);
};

export const User = mongoose.model("User", userSchema);

Mongoose middleware does not run on updateMany or findByIdAndUpdate by default. Either use save() or add { runValidators: true } to the update options.

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