Entity Framework Core Code-First Approach with Seed Data

While working with the EF-Core Code-First approach, we first create the classes for our domain entities. Later, we create the database from our code by using migrations. This is the opposite of the Data-First approach, where we design our database first and then create the classes that match our database design.

How Does Entity Framework Core Code-First Approach Work?

Model First: First, we create a class where we write properties as columns of a data table.
Database Linked Instance: A DbSet<TEntity> can be used to query and save instances of our Model. LINQ queries against a DbSet<TEntity> will be translated into queries against the database.
Seed Data: When you want your data table prefilled with data, the OnModelCreating override method is required.
Adding the Migration: Using Terminal, we can add or update our migrations from classes to the database.

Code Example

Add a Database Connection to your project’s Program.cs file, you can define your connection string in the appsettings.json file.

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddDbContext<ApplicationDbContext>(options =>
    options.UseSqlServer(
        builder.Configuration.GetConnectionString("DefaultConnection")));

var app = builder.Build();

app.Run();

Create your Model with getter and setter properties.

public class Category
{
    [Key]
    public int CategoryId { get; set; }

    [Required]
    public string CategoryName { get; set; }

    public int CategoryDisplayOrder { get; set; }
}

[Key] attribute will indicate the field with identity key and primary key constraints.

[Required] attribute will make the field required and Not-Null field.

Create an ApplicationDbContext class to manage DbContext entities.

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

    public DbSet<Category> Categories { get; set; }
}

Categories will be your table name in the database

To add seed data, prefilled into your Category table.

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

    public DbSet<Category> Categories { get; set; } 
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Category>().HasData(
    	new Category { CategoryId = 1, CategoryName = "Action", CategoryDisplayOrder = 1 },
    	new Category { CategoryId = 2, CategoryName = "SciFi", CategoryDisplayOrder = 2 },
    	new Category { CategoryId = 3, CategoryName = "History", CategoryDisplayOrder = 3 }
        );
    }
}

To add the changes to the database, open a Package Manager Console, Go to Tools > NuGet Package Manager > Package Manager Console.
When Package Manage Console opened; type add-migration <migration_name>, this will create a migration_name.cs file, which you can assess to check if the correct table is being created.
If the migration comment throws error; Make sure you have a package installed Microsoft.EntityFrameworkCore.Tools
To reflect all changes to the database, type update-database.
Now you can check your table in the Database.

You can get the complete code here from this github: https://github.com/arpitsdotnet/EFCoreExample