using System;

using System.Linq;

using System.Collections.Generic;

using Microsoft.EntityFrameworkCore;

​

public interface IStorageCollection<T> : IReadOnlyCollection<T>

{

    void Delete(T entity);

}

​

internal sealed class StorageCollection<T> : HashSet<T>, IStorageCollection<T>

{

    public void Delete(T item) { Remove(item); }

}

​

public class User

{

    public int Id { get; set; }

    public string Name { get; set; }

    public int GroupId { get; set; }

    public Group Group { get; protected set; }

    public void Delete()

    {

        Group = null;

    }

}

​

public class Group

{

    private readonly StorageCollection<User> _users = new StorageCollection<User>();

    public int Id { get; set; }

    public IStorageCollection<User> Users => _users;

    public void AddUser(User user) => _users.Add(user);

}

​

public class AppDbContext : DbContext

{

    public AppDbContext() {}

    public AppDbContext(DbContextOptions<AppDbContext> options) : base(options) {}

​

    public DbSet<Group> Groups { get; set; }

​

    protected override void OnModelCreating(ModelBuilder modelBuilder)

    {

        modelBuilder.Entity<User>()

            .HasOne(u => u.Group)

            .WithMany(g => g.Users).HasForeignKey(u => u.GroupId);

    }

}

​

public class Program

{

    public static void Main()

    {

        var options = new DbContextOptionsBuilder<AppDbContext>().UseSqlite("DataSource=test.db").Options;

        // Arrange

        using (var context = new AppDbContext(options))

        {           

            context.Database.EnsureCreated();

            var group1 = new Group() { Id = 1 };

            group1.AddUser(new User() { Id = 1, Name = "Alice" });

            group1.AddUser(new User() { Id = 2, Name = "Bob" });

            group1.AddUser(new User() { Id = 3, Name = "Connor" });

            var group2 = new Group() { Id = 2 };

            group2.AddUser(new User() { Id = 4, Name = "Eve" });

            context.Groups.Add(group1);

            context.Groups.Add(group2);

            var result = context.SaveChanges();

            Console.WriteLine($"saved: {result}");

        }

        using (var context = new AppDbContext(options))

        {

            var query = context.Groups.SelectMany(g => g.Users);

            query.ToList().ForEach(u => Console.WriteLine($"{u.GroupId} {u.Id} {u.Name}"));

        }

        // Act

        using (var context = new AppDbContext(options))

        {           

            var connor = context.Groups.SelectMany(g => g.Users).Include(u => u.Group).Single(u => u.Id == 3);

            connor.Group.Users.Delete(connor);

            context.SaveChanges();

        }

        // Assert

        Console.WriteLine("===");

        using (var context = new AppDbContext(options))

        {

            var query = context.Groups.SelectMany(g => g.Users).Include(u => u.Group);

            query.ToList().ForEach(u => Console.WriteLine($"{u.Group.Id} {u.Id} {u.Name}"));

        }

    }   

}