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Struggling with nested populate in NestJs using Mongoose? Learn how to correctly fetch your data with this easy-to-follow guide on managing nested relationships.
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Solving NestJs Mongoose Nested Populate Issues: A Step-by-Step Guide

If you are working with NestJs and Mongoose and have faced the frustration of nested populate not working as expected, you are not alone. Many developers encounter challenges when attempting to retrieve related data from MongoDB, especially in complex schemas with multiple levels of relationships.

In this guide, we’ll explore a common problem involving the nested population of documents in Mongoose and provide a clear, organized solution to ensure that all necessary data is retrieved properly.

Understanding the Problem

To set the stage, let’s describe the relationship structure in our example:

Customer: Each customer has an array of persons and a single address.

Person: Each person has a reference to a customer and an address.

Address: Represents the address information for both customers and persons.

The expectation is to populate not only the array of persons associated with a customer but also the addresses for both the customer and the persons. Here’s the main issue: attempts to populate the nested relationships were failing, often returning empty or partial data.

Error Encountered

In one attempt, the code snippet below was used to fetch customer data:

[[See Video to Reveal this Text or Code Snippet]]

However, it resulted in the following type of output:

[[See Video to Reveal this Text or Code Snippet]]

The addresses were not included, leading to confusion and frustration.

The Solution

After thorough investigation and testing, the solution was found to be related to how the Mongoose schema was defined.

Correcting the Schema

The main problem with not being able to retrieve the addresses stemmed from the incorrect schema definitions. Specifically, the @ Prop() decorator’s syntax needed adjustment.

Original Prop Syntax

The initial schema definition for the address field in both Customer and Person models looked like this:

[[See Video to Reveal this Text or Code Snippet]]

This was incorrect.

Updated Prop Syntax

To fix the issue, the prop definitions should be simplified to:

[[See Video to Reveal this Text or Code Snippet]]

And similarly for the array type in the Person model:

[[See Video to Reveal this Text or Code Snippet]]

Final Code Example

Here’s the complete representation of how the schemas should look in your NestJs application:

[[See Video to Reveal this Text or Code Snippet]]

Testing the Fix

After making these changes to the schema, the population logic should work as expected. Now, when you run:

[[See Video to Reveal this Text or Code Snippet]]

You should be able to retrieve a full nested structure that includes both the customer’s address and the addresses linked with each person.

Conclusion

By correctly defining your Mongoose schemas, particularly ensuring the reference definitions are properly set with ObjectId, you can effectively use populate() to retrieve nested relationships in NestJs applications. Avoiding unnecessary complexity will not only make your code cleaner but also make debugging and future maintenance easier.

If you're facing similar challenges, implementing these solutions should pave the way to obtaining the comprehensive data your application needs. Happy coding!