Using Entity Framework Core for Data Access & Relational Mapping/

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Configuring Many-to-Many Relationships

Configuring Many-to-Many Relationships

Learn how to configure many-to-many relationships using default conventions, data annotations, and the fluent API.

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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