Using Entity Framework Core for Data Access & Relational Mapping/

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Configuring One-to-Many Relationships

Configuring One-to-Many Relationships

Learn how to configure a one-to-many relationship using default conventions, data annotations, and fluent API.

We'll cover the following...

Overview

In this lesson, we review the conventions between two entities that automatically result in a one-to-many relationship between their corresponding tables in the database. Later we’ll see how to supplement these conventions using data annotations and the fluent API.

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

There are several conventions by which we establish a one-to-many relationship between two entities. Let’s discuss a couple of them.

Single navigation property

We can create a one-to-many relationship between two entities by including a navigation property in one of the entities. We see this arrangement in the 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 a one-to-many relationship

Click the “Run” button above, then execute the command below in the terminal:

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dotnet ef migrations add SingleNavigationPropertyWithCollectionProperty

After executing the code above, our updated project is 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"
        }
    ]
}
Project illustrating single navigation property with migrations

Note the following in the updated project:

  • There is a relationship between the Employee and Album entities where Employee is the principal entity while Album is the dependent entity.

  • An employee (artist) can have many albums, and we achieve this by including a ...