Software⏱️ 2 min read📅 2026-05-31

How to Fix: Mongoose Unique index not working!

Mongoose unique index not working due to dropDups being set to true. Use dropDups: false instead.

Quick Answer: Set dropDups to false when creating the schema, like this: User = new Schema ({ email: {type: String, index: {unique: true, dropDups: false}} })

📋 Table of Contents

  1. ⚠️ Common Causes
  2. ✅ Best Solutions to Fix It
  3. ✨ Wrapping Up

The issue you're encountering with Mongoose not detecting duplicate values based on a unique index is due to the way MongoDB handles indexing and data types. In MongoDB, an index can be created on a specific field, but it does not guarantee uniqueness across all documents.

⚠️ Common Causes

  • Using the `dropDups` option, which is deprecated in Mongoose 6.0 and later versions.

✅ Best Solutions to Fix It

Method 1: Using a Compound Index

  1. Step 1: Create a compound index on the email field using Mongoose's `index` method.

Method 2: Using the `$unique` Operator

  1. Step 1: Create a unique index on the email field using Mongoose's `index` method with the `$unique` operator.

✨ Wrapping Up

To resolve this issue, you can use either of the methods outlined above. By creating a compound index or using the `$unique` operator, you can ensure that MongoDB detects duplicate values based on the email field.

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