Entity Framework Core with Code First

Code-First with Minimum Configuration

This example uses SQL Server.

Install-Package Microsoft.EntityFrameworkCore.Tools
Install-Package Microsoft.EntityFrameworkCore.SqlServer

public class AppDbContext : DbContext
{
    public DbSet<Hoge> Hoge { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder options)
        => options.UseSqlServer("<接続文字列>"); // <Connection String>
}

public class Hoge
{
    public int Id { get; set; }
    public string Name { get; set; }
}

Add-Migration hoge

PM> Add-Migration hoge
Build started...
Build succeeded.
To undo this action, use Remove-Migration.

Update-Database

PM> Update-Database
Build started...
Build succeeded.
Acquiring an exclusive lock for migration application. See https://aka.ms/efcore-docs-migrations-lock for more information if this takes too long.
Applying migration 'yyyyMMddHHmmss_hoge'.
Done.

In Case of a Web Application

Similar to console applications, code-first can be used in a single project, but it is better to split the projects as follows:

• (A) ASP.NET Core application
• (B) Data access library
• (C) Model (table) definition

The project dependencies are as follows:
(A) ⇒ (B), (C)
(B) ⇒ (C)

If implementing in a single project, simply create each of the following code pieces within that one project.

(A) ASP.NET Core Application

The method remains the same for both Razor Pages and MVC.

• Startup project
• Program.cs (add DbContext DI settings)
• appsettings.json (connection string)

Example: Adding DbContext (PostgreSQL)

// Program.cs
var builder = WebApplication.CreateBuilder(args);

// Add DbContext DI settings
builder.Services.AddDbContext<DefaultDbContext>(options =>
{
    options.UseNpgsql(builder.Configuration.GetDefaultConnectionString());
    options.EnableSensitiveDataLogging();
});

Example: Connection string

// appsettings.json
{
  "ConnectionStrings": {
    "DefaultConnection": "Host=localhost;Database=timecard;Username=postgres;Password=hoge;Port=5432"
  },

(B) Data Access Library

• Migrations (*.cs) auto-generated
• DefaultDbContext.cs
• DefaultDbContext.DbSet.cs
• DefaultDbContext.OnModelCreating.cs
• Database provider (select according to the database used)
  • Microsoft.EntityFrameworkCore.SqlServer
  • Microsoft.EntityFrameworkCore.Sqlite
  • Npgsql.EntityFrameworkCore.PostgreSQL
  • Oracle.EntityFrameworkCore

It is often easier to maintain if you make the DbContext class partial and separate it by purpose.

Example: DefaultDbContext (PostgreSQL)

// DefaultDbContext.cs
public partial class DefaultDbContext : DbContext
{
    public DefaultDbContext(DbContextOptions options)
        : base(options)
    {
        AppContext.SetSwitch("Npgsql.EnableLegacyTimestampBehavior", true);
    }
}

// DefaultDbContext.DbSet.cs
public partial class DefaultDbContext
{
    public DbSet<User> User { get; set; }
    public DbSet<UserCard> UserCard { get; set; }
    public DbSet<TimeCard> TimeCard { get; set; }
}

// DefaultDbContext.OnModelCreating.cs
public partial class DefaultDbContext
{
    protected override void OnModelCreating(ModelBuilder builder)
    {
        base.OnModelCreating(builder);

        builder.Entity<User>(e => { e.HasKey(e => new { e.UserId }); });
        builder.Entity<UserCard>(e => { e.HasKey(e => new { e.UserId, e.CardNumber }); });
        builder.Entity<TimeCard>(e => { e.HasKey(e => new { e.UserId, e.Date }); });

        // Convert names to SnakeCase
        builder.ToSnakeNames();
    }
}

(C) Model (Table) Definition

• Models／Entities (*.cs)

Create the model classes that will become tables here.

(A)'s ViewModel/PageModel: Definition of screen display items
(C)'s Model: Definition of database tables

The key point is to completely separate these roles and establish the dependency (A) ⇒ (C), so that even if there are changes to the screen display specifications, the database table definitions are not directly affected.

Creating Migrations and Applying to the Database

1. Create/Modify models
2. Build the solution
3. Display the Package Manager Console
4. Confirm that "(B) Data Access Library" is selected in the Package Manager Console's "Default project"
5. Confirm that "(A) ASP.NET Core Application" is selected as the startup project
6. Execute the following command

Add-Migration <migration_name>

<migration_name> is arbitrary but must be unique within the Migrations folder. ¹

At this point, a *.cs file defining the changes will be created in the (B) Data Access Library/Migrations folder.

7. Execute the following command:

Update-Database

Model changes will be applied to the database.