Скачать или смотреть How to Join 3 Relation Tables using TypeORM in NestJS

Learn how to effectively perform joins across three related tables in TypeORM with NestJS to retrieve complex data structures without errors.
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Joining 3 Relation Tables using TypeORM in NestJS

If you're working with TypeORM in NestJS and trying to join three related tables but encountering errors, you’re not alone. Many developers face confusion when attempting to manage relationships in their entities, especially when it involves multiple joins. This guide will guide you through a common issue and provide a clear solution to effectively join three related tables: Users, Posts, and Comments.

Understanding the Problem

In the provided scenario, the objective is to retrieve a post, its user, and the comments related to that post, along with the users who made those comments. However, the developer faced an error:

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

This error arises from how relationships are defined between the tables. Let’s break down the relationships and the required entity structure before we delve into the solution.

Table Structures

User Table: Represents the users of the application.

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Post Table: Represents the posts created by users with comments associated with each post.

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Comment Table: Represents comments made on posts.

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The Solution

Now that we understand the structure, let's tackle the error by updating the relationship definitions and correcting the database query.

Step 1: Update the Comment Entity

Update the Comment entity's user relationship to ensure that it uses the correct reference to the user associated with each comment. The code should look as follows:

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This ensures that the userId column in the Comment entity correctly references the User entity.

Step 2: Adjust the Query

After updating the entity, use the following query for joining the tables effectively:

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

Explanation of the Query:

leftJoinAndSelect('post.user', 'user'): Joins the user associated with each post.

leftJoinAndSelect('post.comment', 'comment'): Joins the comments related to each post.

leftJoinAndSelect('comment.user', 'commentedUser'): Joins the user associated with each comment, allowing you to retrieve user details for commenters.

select([...]): Specifies the fields to be returned, including the necessary user information for both posts and comments.

Step 3: Result Structure

With the above adjustments, the resulting data should now look like this:

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

Conclusion

By understanding the relationships between your entities and updating the entity definitions and queries accordingly, you can seamlessly retrieve complex nested data structures without encountering errors. Implementing this solution in your TypeORM with NestJS applications will not only help you avoid common pitfalls but also enhance your ability to manage relational data effectively.

If you have any further questions about TypeORM or complex queries in NestJS, feel free to reach out, and happy coding!