Exercise - Set up a migration

10 minutes

In this unit, you create C# entity classes that map to tables in a local SQLite database. The EF Core migrations feature produces tables from those entities.

A migration provides a way to incrementally update the database schema.

Get the project files

To get started, get the project files. You have some options for how you get the project files:

Use GitHub Codespaces
Clone the GitHub repository

If you have a compatible container runtime installed, you also can use the Dev Containers extension to open the repository in a container with the tools preinstalled.

Use GitHub Codespaces

A codespace is an IDE hosted in the cloud. If you're using GitHub Codespaces, go to the repository in your browser. Select Code, and then create a new codespace in the main branch.

Clone the GitHub repository

If you aren't using GitHub Codespaces, you can clone the project GitHub repo, then open the files as a folder in Visual Studio Code.

Open a command terminal, and then clone the project from GitHub by using the command prompt:
```
git clone https://github.com/MicrosoftDocs/mslearn-persist-data-ef-core
```
Go to the mslearn-persist-data-ef-core folder, and then open the project in Visual Studio Code:
```
cd mslearn-persist-data-ef-core
code .
```

Review the code

Now that you have the project files to work with, let's see what's in the project and review the code.

The ASP.NET Core web API project is located in the ContosoPizza directory. The file paths we refer to in this module are relative to the ContosoPizza directory.
Services/PizzaService.cs is a service class that defines create, read, update, and delete (CRUD) methods. All the methods currently throw System.NotImplementedException.
In Program.cs, PizzaService is registered with the ASP.NET Core dependency injection system.
Controllers/PizzaController.cs is a value for ApiController that exposes an endpoint for HTTP POST, GET, PUT, and DELETE verbs. These verbs call the corresponding CRUD methods on PizzaService. PizzaService is injected into the PizzaController constructor.
The Models folder contains the models that PizzaService and PizzaController use.
The entity models, Pizza.cs, Topping.cs, and Sauce.cs, have the following relationships:
- A pizza can have one or more toppings.
- A topping can be used on one or on many pizzas.
- A pizza can have one sauce, but a sauce can be used on many pizzas.

Build the app

To build the app in Visual Studio Code:

On the Explorer pane, right-click the ContosoPizza directory and select Open in Integrated Terminal.

A terminal pane that is scoped to the ContosoPizza directory opens.
Build the app by using the following command:
```
dotnet build
```
The code should build with no warnings or errors.

Add NuGet packages and EF Core tools

The database engine you work with in this module is SQLite. SQLite is a lightweight, file-based database engine. It's a good choice for development and testing, and it's also a good choice for small-scale production deployments.

Note

As mentioned earlier, database providers in EF Core are pluggable. SQLite is a good choice for this module because it's lightweight and cross-platform. You can use the same code to work with different database engines, such as SQL Server and PostgreSQL. You can even use multiple database engines in the same app.

Before you start, add the required packages:

In the terminal pane, run the following command:
```
dotnet add package Microsoft.EntityFrameworkCore.Sqlite
```
This command adds the NuGet package that contains the EF Core SQLite database provider and all its dependencies, including the common EF Core services.
Next, run this command:
```
dotnet add package Microsoft.EntityFrameworkCore.Design
```
This command adds packages that are required for the EF Core tools.
To finish, run this command:
```
dotnet tool install --global dotnet-ef
```
This command installs dotnet ef, the tool you use to create migrations and scaffolding.

Tip

If dotnet ef is already installed, you can update it by running dotnet tool update --global dotnet-ef.

Scaffold models and DbContext

Now you add and configure a DbContext implementation. DbContext is a gateway through which you can interact with the database.

Right-click the ContosoPizza directory and add a new folder called Data.
In the Data folder, create a new file named PizzaContext.cs. Add the following code to the empty file:
```
using Microsoft.EntityFrameworkCore;
using ContosoPizza.Models;

namespace ContosoPizza.Data;

public class PizzaContext : DbContext
{
    public PizzaContext (DbContextOptions<PizzaContext> options)
        : base(options)
    {
    }

    public DbSet<Pizza> Pizzas => Set<Pizza>();
    public DbSet<Topping> Toppings => Set<Topping>();
    public DbSet<Sauce> Sauces => Set<Sauce>();
}
```
In the preceding code:
- The constructor accepts a parameter of type DbContextOptions<PizzaContext>. The constructor allows external code to pass in the configuration so that the same DbContext can be shared between test and production code, and even be used with different providers.
- The DbSet<T> properties correspond to tables to create in the database.
- The table names match the DbSet<T> property names in the PizzaContext class. You can override this behavior if needed.
- When instantiated, PizzaContext exposes the Pizzas, Toppings, and Sauces properties. Changes you make to the collections that those properties expose are propagated to the database.
In Program.cs, replace // Add the PizzaContext with the following code:
```
builder.Services.AddSqlite<PizzaContext>("Data Source=ContosoPizza.db");
```
The preceding code:
- Registers PizzaContext with the ASP.NET Core dependency injection system.
- Specifies that PizzaContext uses the SQLite database provider.
- Defines a SQLite connection string that points to a local file, ContosoPizza.db.
Note

SQLite uses local database files, so it's okay to hard-code the connection string. For network databases like PostgreSQL and SQL Server, you should always store your connection strings securely. For local development, use Secret Manager. For production deployments, consider using a service like Azure Key Vault.
Also in Program.cs, replace // Additional using declarations with the following code.
```
using ContosoPizza.Data;
```
This code resolves dependencies in the previous step.
Save all your changes. GitHub Codespaces saves your changes automatically.
Build the app in the terminal by running dotnet build. The build should succeed with no warnings or errors.

Create and run a migration

Next, create a migration that you can use to create your initial database.

In the terminal scoped to the ContosoPizza project folder, run the following command to generate a migration for creating the database tables:
```
dotnet ef migrations add InitialCreate --context PizzaContext
```
In the preceding command:
- The migration is named: InitialCreate.
- The --context option specifies the name of the class in the ContosoPizza project, which derives from DbContext.
A new Migrations directory appears in the ContosoPizza project root. The directory contains a <timestamp>_InitialCreate.cs file that describes the database changes to be translated to a Data Definition Language (DDL) change script.
Run the following command to apply the InitialCreate migration:
```
dotnet ef database update --context PizzaContext
```
This command applies the migration. ContosoPizza.db doesn't exist, so this command creates the migration in the project directory.

Tip

All platforms support the dotnet ef tool. In Visual Studio on Windows, you can use the Add-Migration and Update-Database PowerShell cmdlets in the integrated Package Manager Console window.

Inspect the database

EF Core created a database for your app. Next, let's take a look inside the database by using the SQLite extension.

On the Explorer pane, right-click the ContosoPizza.db file and select Open Database.

A SQLite Explorer folder appears in the Explorer pane.
Select the SQLite Explorer folder to expand the node and all its child nodes. Right-click ContosoPizza.db and select Show Table 'sqlite_master' to view the full database schema and constraints that the migration created.
- Tables that correspond to each entity were created.
- Table names were taken from the names of the DbSet properties on the PizzaContext.
- Properties named Id were inferred to be autoincrementing primary key fields.
- The EF Core primary key and foreign key constraint naming conventions are PK_<primary key property> and FK_<dependent entity>_<principal entity>_<foreign key property>, respectively. The <dependent entity> and <principal entity> placeholders correspond to the entity class names.
Note

Like ASP.NET Core MVC, EF Core uses a convention over configuration approach. EF Core conventions shorten development time by inferring the developer's intent. For example, EF core infers a property named Id or <entity name>Id to be the generated table's primary key. If you choose not to adopt the naming convention, you must annotate the property with the [Key] attribute or configure it as a key in the OnModelCreating method of the DbContext.

Change the model and update the database schema

Your manager at Contoso Pizza gives you some new requirements, so you have to change your entity models. In the following steps, you modify the models by using mapping attributes (sometimes called data annotations).

In Models\Pizza.cs, make the following changes:
1. Add a using directive for System.ComponentModel.DataAnnotations.
2. Add a [Required] attribute before the Name property to mark the property as required.
3. Add a [MaxLength(100)] attribute before the Name property to specify a maximum string length of 100.
Your updated Pizza.cs file should look like the following code:
```
using System.ComponentModel.DataAnnotations;

namespace ContosoPizza.Models;

public class Pizza
{
    public int Id { get; set; }

    [Required]
    [MaxLength(100)]
    public string? Name { get; set; }

    public Sauce? Sauce { get; set; }

    public ICollection<Topping>? Toppings { get; set; }
}
```
In Models\Sauce.cs, make the following changes:
1. Add a using directive for System.ComponentModel.DataAnnotations.
2. Add a [Required] attribute before the Name property to mark the property as required.
3. Add a [MaxLength(100)] attribute before the Name property to specify a maximum string length of 100.
4. Add a bool property named IsVegan.
Your updated Sauce.cs file should look like the following code:
```
using System.ComponentModel.DataAnnotations;

namespace ContosoPizza.Models;

public class Sauce
{
    public int Id { get; set; }

    [Required]
    [MaxLength(100)]
    public string? Name { get; set; }

    public bool IsVegan { get; set; }
}
```
In Models\Topping.cs, make the following changes:
1. Add using directives for System.ComponentModel.DataAnnotations and System.Text.Json.Serialization.
2. Add a [Required] attribute before the Name property to mark the property as required.
3. Add a [MaxLength(100)] attribute before the Name property to specify a maximum string length of 100.
4. Add a decimal property named Calories immediately after the Name property.
5. Add a Pizzas property of type ICollection<Pizza>?. This change makes Pizza-Topping a many-to-many relationship.
6. Add a [JsonIgnore] attribute to the Pizzas property.
  
  Important
  
  This attribute prevents Topping entities from including the Pizzas property when the web API code serializes the response to JSON. Without this change, a serialized collection of toppings would include a collection of every pizza that uses the topping. Each pizza in that collection would contain a collection of toppings, which each would again contain a collection of pizzas. This type of infinite loop is called a circular reference and can't be serialized.
Your updated Topping.cs file should look like the following code:
```
using System.ComponentModel.DataAnnotations;
using System.Text.Json.Serialization;

namespace ContosoPizza.Models;

public class Topping
{
    public int Id { get; set; }

    [Required]
    [MaxLength(100)]
    public string? Name { get; set; }

    public decimal Calories { get; set; }

    [JsonIgnore]
    public ICollection<Pizza>? Pizzas { get; set; }
}
```
Save all your changes and run dotnet build.
Run the following command to generate a migration for creating the database tables:
```
dotnet ef migrations add ModelRevisions --context PizzaContext
```
This command creates a migration named: ModelRevisions.

Note

You'll see this message: An operation was scaffolded that may result in the loss of data. Please review the migration for accuracy. This message appeared because you changed the relationship from Pizza to Topping from one-to-many to many-to-many, which requires that an existing foreign-key column is dropped. Because you don't yet have any data in your database, this change isn't problematic. However, in general, it's a good idea to check the generated migration when this warning appears to make sure that the migration doesn't delete or truncate any data.
Run the following command to apply the ModelRevisions migration:
```
dotnet ef database update --context PizzaContext
```
On the title bar of the SQLite Explorer folder, select the Refresh Databases button.
In the SQLite Explorer folder, right-click ContosoPizza.db. Select Show Table 'sqlite_master' to view the full database schema and constraints.

Important

The SQLite extension reuses open SQLite tabs.
- A PizzaTopping join table was created to represent the many-to-many relationship between pizzas and toppings.
- New fields were added to Toppings and Sauces.
  - Calories is defined as a text column because SQLite doesn't have a matching decimal type.
  - Similarly, IsVegan is defined as an integer column. SQLite doesn't define a bool type.
  - In both cases, EF Core manages the translation.
- The Name column in each table was marked not null, but SQLite doesn't have a MaxLength constraint.
Tip

EF Core database providers map a model schema to the features of a specific database. Although SQLite doesn't implement a corresponding constraint for MaxLength, other databases like SQL Server and PostgreSQL do.
In the SQLite Explorer folder, right-click the _EFMigrationsHistory table and select Show Table. The table contains a list of all migrations that are applied to the database. Because you ran two migrations, there are two entries: one for the InitialCreate migration and another for ModelRevisions.