Скачать или смотреть Creating EF Core 3 Relationships in Old Databases Without Referential Integrity

Learn how to define relationships in Entity Framework Core 3 for an existing database without foreign keys, using navigation properties and Fluent API.
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Navigating Entity Framework Core 3 Without Referential Integrity

Entity Framework (EF) Core 3 is a powerful tool for interacting with databases in .NET applications, but what if you're dealing with an old database that lacks any defined foreign keys? You've got a situation where you need to create relationships in your code model for navigation and querying purposes, even though your database doesn't support referential integrity. In this guide, we'll guide you through how to establish these relationships using EF Core 3, making your data access simpler and more efficient.

Understanding the Challenge

You’re working with an existing database, possibly set up for a legacy application. Because of its age and design, you cannot alter the schema to add foreign keys or primary keys as one would in a new database design. This limitation can hinder your ability to create navigational relationships in your application code, which is crucial for leveraging LINQ queries effectively.

The Solution Outline

Fortunately, EF Core 3 offers a way to address this issue using a combination of Fluent API configurations and navigation properties. This allows you to define relationships within your code that EF can recognize, even though those relationships don't exist in the database schema.

Setting Up Your Entities

Based on the provided code snippets, let's break this down step by step:

1. Define Your Entities

Assuming you have two entities: UsagerEw and RefreshToken, start by defining the code model collections.

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

2. Configurate Relationships with Fluent API

In your DbContext, you can use the Fluent API to set up the relationship between RefreshToken and UsagerEw.

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

Notes on Virtual Properties

Including the virtual keyword is optional but recommended if you want EF Core to support lazy loading. By making navigation properties virtual, EF Core has the ability to automatically load related data when accessed.

3. Address Missing Columns

If you encounter an error such as SqlException: Invalid column name 'UsagerEWId', it indicates that your RefreshToken class is referencing a foreign key column that doesn't exist in your database. Ensure that any foreign key properties you have created actually match the existing structure of your database.

Other Considerations

Configuration Options: You can configure relationships in multiple ways:

Data Annotations: Simple attributes within your entity classes.

Fluent API via OnModelCreating method.

IEntityTypeConfiguration classes that keep your configurations organized for larger schemas.

Querying without Navigation Properties: If you don’t require navigation properties for your logic, it’s enough to simply map the foreign keys without the relationships in code.

Conclusion

Managing database relationships in Entity Framework Core 3 without support for referential integrity can be challenging, but with the powerful capabilities of Fluent API and a strong understanding of how to set up your entities, it can be achieved effectively. The ability to leverage LINQ queries for rich data interactions is an essential part of developing robust applications. Hopefully, this guide has illuminated the path forward for your Entity Framework Core 3 endeavors, even when dealing with legacy databases.

Feel free to ask any questions or share your experiences dealing with similar scenarios in EF Core!