code first: many-to-many example

different files.cs

AppDbContext.cs:

using EFCoreCodeFirst.Models;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;

namespace EFCoreCodeFirst
{
    public class AppDbContext : DbContext
    {
        public DbSet<User>? Users => Set<User>();
        public DbSet<Order>? Orders => Set<Order>();

        public DbSet<Product>? Products => Set<Product>();
        public DbSet<Category>? Categories => Set<Category>();

        public DbSet<Person>? People { get; set; }
        public DbSet<Hobby>? Hobbies { get; set; }
        public DbSet<PersonHobby>? PersonHobbies { get; set; }

        public AppDbContext()
        {

        }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            if (!optionsBuilder.IsConfigured)
            {
                var configuration = new ConfigurationBuilder()
                    .SetBasePath(Directory.GetCurrentDirectory())
                    .AddJsonFile("appsettings.json")
                    .Build();

                optionsBuilder.UseSqlServer(configuration.GetConnectionString("DefaultConnection"));
            }
        }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            base.OnModelCreating(modelBuilder);

            // налаштування зв'язку М-М
            modelBuilder.Entity<PersonHobby>()
                .HasKey(ph => new { ph.PersonId, ph.HobbyId });

            modelBuilder.Entity<PersonHobby>()
                .HasOne(ph => ph.Person)
                .WithMany(p => p.PersonHobbies)
                .HasForeignKey(ph => ph.PersonId);

            modelBuilder.Entity<PersonHobby>()
                .HasOne(ph => ph.Hobby)
                .WithMany(h => h.PersonHobbies)
                .HasForeignKey(ph => ph.HobbyId);
        }
    }
}

=============================================================================================

Person.cs:

namespace EFCoreCodeFirst.Models
{
    public class Person
    {
        public int PersonId { get; set; } // Primary Key
        public string? Name { get; set; }

        public ICollection<PersonHobby>? PersonHobbies { get; set; } 
    }
}

=============================================================================================

Hobby.cs:

namespace EFCoreCodeFirst.Models
{
    public class Hobby
    {
        public int HobbyId { get; set; } // Primary Key
        public string? Name { get; set; }

        public ICollection<PersonHobby>? PersonHobbies { get; set; }
    }
}

=============================================================================================

PersonHobby.cs:

namespace EFCoreCodeFirst.Models
{
    public class PersonHobby
    {
        public int PersonId { get; set; } // зовнішній ключ до Person
        public Person? Person { get; set; }

        public int HobbyId { get; set; } // зовнішній ключ до Hobby
        public Hobby? Hobby { get; set; }
    }
}

=============================================================================================

Program.cs:

using EFCoreCodeFirst.Models;
using Microsoft.EntityFrameworkCore;

namespace EFCoreCodeFirst
{
    class Program
    {
        static async Task Main()
        {
            Console.Title = "Міграції";
            Console.OutputEncoding = System.Text.Encoding.UTF8;

            await using var db = new AppDbContext();

            // await db.Database.EnsureDeletedAsync(); // видаляє БД, якщо вона існувала

            Console.WriteLine("Перевірка та застосування міграцій...");
            await db.Database.MigrateAsync();
            Console.WriteLine("Міграції застосовані (або вже були актуальні).\n");

            var hobby1 = new Hobby { Name = "Футбол" };
            var hobby2 = new Hobby { Name = "Малювання" };
            var hobby3 = new Hobby { Name = "Читання" };
            var hobby4 = new Hobby { Name = "Подорожі" };
            var hobby5 = new Hobby { Name = "Кулінарія" };

            db.Hobbies?.AddRange(hobby1, hobby2, hobby3, hobby4, hobby5);

            var person1 = new Person { Name = "Олександр" };
            var person2 = new Person { Name = "Руслан" };
            var person3 = new Person { Name = "Максим" };
            var person4 = new Person { Name = "Карина" };
            var person5 = new Person { Name = "Аліса" };

            db.People?.AddRange(person1, person2, person3, person4, person5);

            db.PersonHobbies?.AddRange(
                new PersonHobby { Person = person1, Hobby = hobby1 },
                new PersonHobby { Person = person1, Hobby = hobby3 },
                new PersonHobby { Person = person2, Hobby = hobby2 },
                new PersonHobby { Person = person2, Hobby = hobby4 },
                new PersonHobby { Person = person3, Hobby = hobby5 },
                new PersonHobby { Person = person3, Hobby = hobby1 },
                new PersonHobby { Person = person4, Hobby = hobby2 },
                new PersonHobby { Person = person4, Hobby = hobby5 },
                new PersonHobby { Person = person5, Hobby = hobby3 },
                new PersonHobby { Person = person5, Hobby = hobby4 }
            );

            db.SaveChanges();

            using (var context = new AppDbContext())
            {
                var peopleWithHobbies = context.People?.Include(p => p.PersonHobbies!).ThenInclude(ph => ph.Hobby).ToList();

                foreach (var person in peopleWithHobbies!)
                {
                    Console.WriteLine($"{person.Name}");
                    foreach (var personHobby in person.PersonHobbies!)
                    {
                        Console.WriteLine($" - Захоплення: {personHobby.Hobby?.Name}");
                    }
                }
            }
        }
    }
}