Introduction to Object-Relational Mapping

Currently our app only saves data while it is running. Once you end the session and close the program, all of the data you provided disappears. We can make that data persistent by connecting your application to a database. This will retain the data even if the application is not running, and allow you to access it when you open a new session.

Let’s connect our MVC application to a relational database and add persistent data storage to our apps. To do so, we need to use object-relational mapping.

Object-Relational Mapping or ORM is a technique for converting data between C# objects and relational databases. ORM converts data between two incompatible type systems (C# and MySQL), such that each model class becomes a table in our database and each instance a row of the table.

To make ORM work in our C# applications, we need an object-relational mapper to convert between C# and MySQL. When we create and configure a new model class to be stored in a database, a mapper creates a MySQL query to make the corresponding table.

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public Events codeWithPride = new Events("Code With Pride", "Creating community for LGBTQIA+ technologists and allies", "[email protected]")

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INSERT INTO Events (Name, Description, ContactEmail)
VALUES ("Code With Pride",Creating community for LGBTQIA+ technologists and allies", "info@launchcode.org");

ORM in ASP.NET

One of the most widely used object-relational mappers available for C# and ASP.NET Core is Entity Framework Core. This framework makes use of data layers. When we learned about models , we learned that data layers add abstraction between models and the data we want to store. With Entity FrameworkCore, data layers take the form of classes that extend DbContext.

Before we add persistence to our data, it’s important to remember that: Models are NOT persistent data stores, and relational databases do NOT shape the C# objects we will be using. We want to make sure that the two remain separate.

Adding ORM to CodingEvents

To work with EF Core, we need to add it to the project.

Currently, Visual Studio 2022 (Windows and Mac) is typically installed with EF 6.0.X.

Verify EF Core Tools are Present

If you downloaded Visual Studio 2022 or later, let’s verify EF Core’s presence and version.

You can test that it has been installed in Visual Studio by running the following in your terminal.

1. cd your way down into the project folders. Verify your location by running the ls command. You should see all the folders within your project.

   student-computer:CodingEvents student$ ls
   CodingEvents.csproj               ViewModels
   Controllers                       Views
   Data                              appsettings.Development.json
   Models                            appsettings.json
   Program.cs                        bin
   Properties                        obj
   Startup.cs                        wwwroot

2. When you are this level run the following command:

   dotnet ef

   You should see the followng output:

      student-computer:CodingEventsDemo student$ dotnet ef
   
               _/\__
         ---==/    \\
   ___  ___   |.    \|\
   | __|| __|  |  )   \\\
   | _| | _|   \_/ |  //|\\
   |___||_|       /   \\\/\\
   
   Entity Framework Core .NET Command-line Tools 6.0.X
   
   //code continues ...

Troubleshooting EF Core Tools

If you are not able to see the Entity Framework Core logo, then try the following steps to troubleshoot the issue.

1. Open a terminal window using your terminal app outside of Visual Studio.

2. Open a terminal and run:

   dotnet tool install -g dotnet-ef

   This command installs a set of command-line tools for working with EF globally, which means it will be available for any ASP.NET project we use in the future. We will use the tools provided by this package to update our database schema after adding or changing model classes.

3. Once you have taken these steps, you are ready to set up the appropriate models and controllers for the application. We’ll do that in the next section.

4. To test that this install worked, run dotnet ef. The output should be a message displaying basic EF tool commands and options

   Note

   Note for Mac users only

   For these tools to be accessible from the command line, they must be within your user path. We create or update your bash profile. Your bash profile is a text file that you can add any paths needed. You may add to this as you continue on your programming journey.

   1. Open your ~/.bash_profile with this command:

      open ~/.bash_profile

      The ~ symbol is shorthand for your home directory, which is the directory you are in when you open a new terminal window.

   2. Add the following line to the very bottom of your profile:

      export PATH="$PATH:$HOME/.dotnet/tools/"

   3. Save and close the file. Then close your terminal window and open a new one, so that the changes can take effect.

   4. To test that this install worked, run dotnet ef. The output should be a message displaying basic EF tool commands and options.

Once you have taken these steps, you are ready to set up the appropriate models and controllers for the application. We’ll do that in the next section.

Check Your Understanding