remove-links-during-patch-bug.cs

using AutoMapper;
using Microsoft.Data.Sqlite;
using Microsoft.EntityFrameworkCore;

namespace EntityFrameworkMappingBug;

internal class Program
{
    private static async Task Main(string[] args)
    {

        var config = new MapperConfiguration(cfg =>
        {
            cfg.CreateMap<UpdateUserGroupRequest, UpdateUserGroupDTO>();
            cfg.CreateMap<UpdateUserGroupDTO, UserGroup>();
        });
        var mapper = new Mapper(config);

        var connection = new SqliteConnection("Filename=:memory:");
        connection.Open();

        var contextOptions = new DbContextOptionsBuilder<MyDbContext>()
            .UseSqlite(connection)
            .Options;

        using var context = new MyDbContext(contextOptions);

        await context.Database.EnsureCreatedAsync();

        var userGroup = new UserGroup
        {
            Name = "FULL",
            Users = new List<UserGroupUserLink>
            {
                new() {
                    User = new User
                    {
                        Name = "Jan Kowalski"
                    }
                }
            }
        };

        await context.Groups.AddAsync(userGroup);
        await context.SaveChangesAsync();

        Console.WriteLine($"Liczba Groups: {context.Groups.Count()} (oczekiwana 1).");
        Console.WriteLine($"Liczba Users: {context.Users.Count()} (oczekiwana 1).");
        Console.WriteLine($"Liczba Links: {context.Links.Count()} (oczekiwana 1).");

        Console.WriteLine("Wykonywana operacja PATCH.");

        Console.WriteLine("Pobieranie grupy z bazy danych.");
        var dbUserGroup = context.Groups.First();

        var request = new UpdateUserGroupRequest
        {
            Name = "FULL 2"
        };

        Console.WriteLine("Mapowanie requestu na DTO.");

        var requestDTO = mapper.Map<UpdateUserGroupDTO>(request);

        Console.WriteLine("Mapowanie DTO na grupę.");

        mapper.Map(requestDTO, dbUserGroup);

        Console.WriteLine("Update encji.");

        context.Update(dbUserGroup);
        await context.SaveChangesAsync();

        Console.WriteLine($"Liczba UserGroups: {context.Groups.Count()} (oczekiwana 1).");
        Console.WriteLine($"Liczba Users: {context.Users.Count()} (oczekiwana 1).");
        Console.WriteLine($"Liczba Links: {context.Links.Count()} (powinna być 1, jest 0 - błąd!).");
    }
}

public class UpdateUserGroupRequest
{
    public string Name { get; set; } = null!;
}

public class UpdateUserGroupDTO
{
    public string Name { get; set; } = null!;

    // VERY SNEAKY!
    public List<UserGroupUserLink> Users { get; set; } = null!;
}

public class UserGroup
{
    public int Id { get; set; }
    public string Name { get; set; }
    public List<UserGroupUserLink> Users { get; set; }
}

public class UserGroupUserLink
{
    public int Id { get; set; }
    public int UserGroupId { get; set; }
    public UserGroup UserGroup { get; set; }
    public int UserId { get; set; }
    public User User { get; set; }
}

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

public class MyDbContext : DbContext
{
    public MyDbContext(DbContextOptions<MyDbContext> options) : base(options) { }
    public DbSet<User> Users { get; set; }
    public DbSet<UserGroup> Groups { get; set; }
    public DbSet<UserGroupUserLink> Links { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<UserGroupUserLink>()
            .HasOne(p => p.UserGroup)
            .WithMany(b => b.Users)
            .HasForeignKey(p => p.UserGroupId);

        modelBuilder.Entity<UserGroupUserLink>()
            .HasOne(p => p.User)
            .WithMany()
            .HasForeignKey(p => p.UserId);
    }
}