Скачать или смотреть Resolving the Issue: discriminatorKey Not Automatically Populating in NestJS with Mongoose

Discover how to fix the issue of the `discriminatorKey` not being automatically populated in NestJS with Mongoose, and learn the importance of injecting the correct model.
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Understanding the Issue: discriminatorKey Not Automatically Populating

When working with NestJS and Mongoose, developers often face various hurdles, especially when dealing with complex data structures like discriminators. A common issue that arises is when the discriminatorKey - in this case, the kind field in our schemas - doesn't seem to populate automatically. This can lead to confusion and errors when trying to save documents. Let's delve deeper into this problem and see how we can effectively resolve it.

What is a Discriminator in Mongoose?

In Mongoose, a discriminator allows you to create multiple models on top of a single schema. This is particularly useful when you have documents that share a common set of fields but also have specific fields of their own. The discriminatorKey is used to identify which discriminator is being used for each document.

For instance, if you have a base User schema and derived schemas like Fan, the kind field acts as a discriminator key that indicates what type of user is being represented.

The Problem: discriminatorKey Not Being Filled

Despite correctly setting up your schemas, you may find that the kind field is missing from documents upon saving. This can frustrate developers trying to utilize discriminators effectively.

Example Code Structure

Here's a quick overview of the code structure we are working with:

Database Module:

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

User Schema:

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

Fan Schema:

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

The Root of the Problem: Model Injection Issue

After careful inspection, it was discovered that the issue was due to injecting the wrong model within the constructor. Here is how that mistake typically looks:

Wrong Model Injection

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

When you inject the User model instead of the specific Fan model, Mongoose fails to recognize which discriminator you are trying to work with, and thus, the discriminatorKey isn’t populated.

The Correct Approach

To resolve this issue, ensure that you are injecting the correct model for the operation you want to perform. The correct code should look like this:

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

Saving a Document

When you save a Fan object now, you'll find that the kind field is filled automatically:

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

Conclusion

In conclusion, handling discriminators in NestJS with Mongoose may present challenges, especially when models are incorrectly injected. Remember to always inject the right model to ensure that the discriminatorKey is populated automatically. Following this guide should help you navigate and resolve similar issues in your development journey.

Feel free to reach out with any questions or further clarifications — happy coding!