Simplifying EF Core Joins on Sub Tables for Item and User Entities

Описание: Learn how to effectively join sub tables in `EF Core` using navigations for `Item` and `User` entities without complicating your queries.
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This video is based on the question https://stackoverflow.com/q/67227855/ asked by the user 'Richard Harris' ( https://stackoverflow.com/u/14965516/ ) and on the answer https://stackoverflow.com/a/67228033/ provided by the user 'Johnathan Barclay' ( https://stackoverflow.com/u/8126362/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem: Joining Sub Tables in EF Core

When working with Entity Framework Core (EF Core), developers often encounter situations where multiple tables need to be joined, particularly when dealing with relationships in a database. A common scenario arises when you have three classes, namely Item, User, and UserItem, where UserItem acts as a join table between User and Item.

If you find yourself struggling to retrieve Item records for a User, despite having defined the relationships properly in your code, you are not alone. Let's take a closer look at how to streamline these joins in EF Core effectively.

Existing Classes Structure

Here’s a quick look at how the classes are currently structured:

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Problematic Querying

Although you can retrieve UserItem collections from your User and Item entities, the process of querying from User to Item feels cumbersome. For instance, the following SQL query achieves this by joining tables:

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In C# , you might have resorted to writing complex LINQ queries like this:

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This approach, while functional, often feels unnecessarily complicated due to the explicit joining of the UserItem table.

Streamlining the Join Process

To simplify your queries in EF Core, it is essential to establish direct navigational properties in the UserItem class itself. Here’s how you can enhance your existing model:

Step 1: Update UserItem Class

Add navigations to User and Item within the UserItem class:

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Step 2: Refactor the Query

With this new structure, your query can be greatly simplified. You can utilize Include and ThenInclude methods to get the associated Items directly from Users:

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This approach effectively retains the relations between tables but makes the querying process more straightforward and manageable.

Optional: Omit UserItem for .NET 5 or Later

If you use .NET 5 or a later version, you can take this simplification a step further by omitting the UserItem class entirely. You could define your Item and User classes as follows:

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This would allow a much cleaner query:

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Conclusion

By making these modifications, you can optimize the way you join your User and Item entities in EF Core. This not only improves the readability and maintainability of your code but also enhances performance. Adapting navigational properties allows for simpler queries that align more closely with the expected database interactions, making your development experience smoother.

In conclusion, if you find yourself querying join tables, consider the above structural changes to simplify your code and improve efficiency in your EF Core applications.