Скачать или смотреть Resolving the Null Foreign Key Issue in EF Core with Self-Referencing Entities

Learn how to resolve the issue where EF Core retrieves entities with null properties for foreign keys pointing to the same model in your ASP.NET Core application.
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This video is based on the question https://stackoverflow.com/q/63608441/ asked by the user 'khargoosh' ( https://stackoverflow.com/u/4267590/ ) and on the answer https://stackoverflow.com/a/63608575/ provided by the user 'Gabriel Llorico' ( https://stackoverflow.com/u/2122217/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving the Null Foreign Key Issue in EF Core with Self-Referencing Entities

In the world of data management, it is common to encounter references within models that point back to themselves. This can create challenges, particularly when working with Entity Framework Core (EF Core). One common issue arises when retrieving a record, and you find that a property representing a foreign key to the same entity is returning null. This guide dives into why this happens and how you can effectively resolve the issue.

Understanding the Problem

You have a model class representing a Customer, which can optionally reference a "Parent" customer through a foreign key. Your goal is to retrieve a specific customer along with its parent data. However, upon querying, you find that the Parent property of the returned customer object is null, even though this data exists in the underlying database.

Example Model and Database Structure

Consider the following Customer model:

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And the associated database table:

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The Issue in Code

When you try to fetch a customer in your controller:

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The retrieved customer object has a null Parent property, leading to a Null Reference Exception (NRE) while creating the CustomerViewModel.

Solution: Fetching Parent Data Correctly

To ensure that the Parent data is included when you query the customer, you can modify the query to explicitly include the Parent property. Here’s how to go about it.

Using Include Method

The easiest way to fetch related data is to use the Include method:

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This command tells EF Core to also include the Parent object when retrieving the customer.

Alternative Approach: Manual Parent Retrieval

If including the Parent with the original query does not work as expected, you can manually retrieve it as shown below:

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Conclusion

When working with self-referencing entities in EF Core, it is crucial to ensure that related data is included in your queries to avoid receiving null for properties that should have values. By either using the Include method or retrieving the foreign key manually, you can effectively manage these situations, leading to a more robust and error-free application.

Take these lessons to build better data handling strategies in your ASP.NET Core applications and avoid common pitfalls with entity relationships.