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Using Entity Framework Core for Data Access & Relational Mapping

Configuring Many-to-Many Relationships

Explore how to configure many-to-many relationships between entities in EF Core. Learn to use collection navigation properties, manage join tables, and apply default conventions. Understand how to override these conventions with data annotations or the fluent API to customize your database schema effectively.

We'll cover the following...

Overview

In this lesson, we’ll look at the conventions between two entities that result in a many-to-many relationship between their corresponding tables in the database. We’ll also look at how to supplement these conventions using data annotations and the fluent API.

Many-to-many relationships utilize a collection navigation property at both ends. In relational databases, a join table represents a many-to-many relationship in the database. This table includes the foreign keys of both tables.

Note: The commands in this lesson generate code and files. Through the terminal, we can navigate to these files by using relevant Linux commands such as ls to view a list of files and directories), cd to change directories, and cat to view file contents. A SPA widget showing the updated project with the generated files is also available. Also, note that EF Core uses a timestamp within the generated file names. We represent these names with xxx.

Default conventions

By convention, when EF Core discovers a collection navigation property at both ends of entities, it creates an entity type internally to represent a join table. The entity type is called a join entity type. EF Core uses two one-to-many relationships on the join entity type to represent the many-to-many relationship.

We demonstrate configuring a many-to-many relationship using the C# project below:

{
    "version": "0.2.0",
    "configurations": [
        {
            // Use IntelliSense to find out which attributes exist for C# debugging
            // Use hover for the description of the existing attributes
            // For further information visit https://github.com/OmniSharp/omnisharp-vscode/blob/master/debugger-launchjson.md
            "name": ".NET Core Launch (console)",
            "type": "coreclr",
            "request": "launch",
            "preLaunchTask": "build",
            // If you have changed target frameworks, make sure to update the program path.
            "program": "${workspaceFolder}/bin/Debug/net6.0/Relationships.dll",
            "args": [],
            "cwd": "${workspaceFolder}",
            // For more information about the 'console' field, see https://aka.ms/VSCode-CS-LaunchJson-Console
            "console": "internalConsole",
            "stopAtEntry": false
        },
        {
            "name": ".NET Core Attach",
            "type": "coreclr",
            "request": "attach"
        }
    ]
}
Configuring many-to-many relationships (default conventions)

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