Скачать или смотреть How to Fix the Mongoose Virtual Property Not Working in findById Queries

Discover effective solutions for resolving the issue of `Mongoose virtual properties` not functioning correctly when using `findById`.
---
This video is based on the question https://stackoverflow.com/q/65894263/ asked by the user 'svoruganti' ( https://stackoverflow.com/u/13708703/ ) and on the answer https://stackoverflow.com/a/65894498/ provided by the user '王仁宏' ( https://stackoverflow.com/u/11303576/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: mongoose virtual property not working in findById property

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
How to Fix the Mongoose Virtual Property Not Working in findById Queries

When working with Mongoose, many developers experience challenges, particularly when dealing with virtual properties. One common issue is when a virtual property is found to be undefined while fetching data using the findById method. This guide will detail why virtual fields may not work as expected and provide you with several solutions to fix the problem.

Understanding the Problem

Virtual properties in Mongoose are used to create fields that are not stored in the database but can be derived from actual fields. These virtual properties can be valuable for formatting data for output without duplicating information in your database.

Here’s a specific problem scenario:

You have defined a virtual field (e.g., avg_satis) in your schema to calculate the average satisfaction from related sessions.

However, when you try to access this virtual property using findById, it returns undefined.

Example of the Problematic Code

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

The issue arises because virtual properties do not exist in the database. Therefore, they cannot be directly queried or retrieved in the way you might expect.

Solution Breakdown

Here's a structured approach to fix the undefined virtual property issue.

1. Understanding Virtual Properties

As a developer, it's essential to note that:

Virtuals are not stored in the database. They are a shorthand used for manipulations or computations based on the data stored.

The virtual method is unique because it does not directly return a value from the database; instead, it calculates and provides a derived value.

2. Returning Promises in Virtuals

If you want your virtual to return a value asynchronously (and you want to calculate the average satisfaction), you could modify it to return a Promise:

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

3. Using Instance Methods Instead

A more reliable approach might be to define a method in your schema that can compute the average satisfaction:

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

4. How to Call the Method

To use this method effectively:

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

5. Exploring Aggregation Pipelines

For more advanced computations, consider using Mongoose’s aggregate function, which can help in obtaining more complex derived fields directly from MongoDB.

Conclusion

Virtual properties are powerful but can lead to confusion if not understood properly. The main takeaway is that virtuals cannot be treated like standard fields fetched from the database. Using instance methods or asynchronous promises can greatly enhance your Mongoose queries, allowing you to obtain the derived data you need effectively.

By following the solutions provided in this guide, you can confidently address and resolve the issue of virtual properties not functioning as intended. Happy coding!