The Hidden Limitation of Mongoose Discriminators That Surprises Many Developers

While working on a laundry service project using MongoDB and Mongoose, I ran into a surprisingly common issue E11000 duplicate key error collection: LaundryHouse index: name_1 dup key: { name: "nylon" } and it turns out, I wasn’t alone. This limitation happens when using Mongoose discriminators in combination with unique: true indexes. It caused a duplicate key error that initially seemed like a mistake in my code but it wasn’t.

Mongoose discriminators allow you to define multiple schemas (like ClotheType, PackingType, etc.) that share the same parent model and collection. In my case, all these types are stored in the same LaundryHouse collection. Everything worked fine until I tried to create a PackingType with the same name (e.g., "nylon") for two different laundry businesses. MongoDB threw a duplicate key error, complaining that the value "nylon" already existed even though it was for separate businesses.

At first glance, this doesn’t make sense. But under the hood, the issue comes from how Mongoose and MongoDB handle indexes. When you use unique: true on a field like name, Mongoose creates a global unique index on that field in the shared collection. MongoDB doesn’t care about your business logic or relationships if two documents have the same name, it throws an error, even if they belong to different entities.

This behavior is completely normal when using Mongoose discriminators. It’s not a bug or a flaw in your logic it’s simply a side effect of working with shared collections and automatic indexing. Many developers run into this when building multi-tenant applications or when trying to enforce field-level uniqueness in isolated contexts.

The correct approach is to replace the global unique: true with a compound unique index. For example, in my case, I used packingTypeSchema.index({ laundryBusiness: 1, name: 1 }, { unique: true });. This ensures that each name is unique per laundryBusiness, but allows different businesses to use the same name like "nylon" without any conflict.

So if you’re using Mongoose discriminators and facing duplicate key errors, check your indexes especially if you're using unique: true on shared fields. And remember, this is a known limitation, not a mistake. The fix is simple, and it will save you a lot of debugging time.

Hope this helps someone out there struggling with the same issue!