Issue when trying to update some entities that are in a many to many relationships with a favourite child

[akilin]

Bug description

Hi, I have encountered what I think is a bug when trying to update some entities that are in a many to many relationships with a favourite child.

I have 3 entities: User, Group, GroupMember.
Group also has a GroupOwner field referencing GroupMember.

I am trying to perform an update that will change list of members & change the ownership of the group.
Both operations work fine when done in isolation. But attempting to perform them in one go results in an error, and a weird one at that.

The code below throws:

System.InvalidOperationException: 'The property 'Group.Id' is defined as read-only after it has been saved,
but its value has been modified or marked as modified.'

even tho Group.Id was never touched.

group.Members = [new() { UserId = 1, GroupId = 1 }];
group.GroupOwnerId = 1;
ctx.SaveChanges();

---

repo with reproduction code:
https://github.com/akilin/demos/tree/master/ef-many-to-many-updates

steps to reproduce locally:

git clone https://github.com/akilin/demos
cd demos
cd .\ef-many-to-many-updates\
docker-compose up -d
dotnet run

Your code

Full code in a single file can be seen here (this is a simplified reproduction of what we are observing in our real app):
https://github.com/akilin/demos/blob/master/ef-many-to-many-updates/Program.cs

using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Logging;
using System;
using System.Collections.Generic;
using System.Linq;

namespace ef_many_to_many_updates
{
    static class Program
    {
        static void Main(string[] args)
        {
            // reset db
            using var ctx = new AppContext();
            ctx.Database.EnsureDeleted();
            ctx.Database.EnsureCreated();

            // arrange
            ctx.Groups.Add(new Group { Id = 1 });
            ctx.Users.Add(new User { Id = 1 });
            ctx.GroupMembers.Add(new GroupMember { UserId = 1, GroupId = 1 });
            ctx.SaveChanges();

            ctx.ChangeTracker.Clear();

            // act
            var group = ctx.Groups.Include(x => x.Members).Single();
            //either of the 2 lines below work fine on their own. but combined - they cause the issue
            group.Members = [new() { UserId = 1, GroupId = 1 }];
            group.GroupOwnerId = 1;
            ctx.SaveChanges();
        }
    }

    public class User
    {
        public int Id { get; set; }
        public ICollection<GroupMember> Groups { get; set; } = null!;
    }

    public class Group
    {
        public int Id { get; set; }
        public int? GroupOwnerId { get; set; }
        public GroupMember? GroupOwner { get; set; }
        public ICollection<GroupMember> Members { get; set; } = null!;
    }

    public class GroupMember
    {
        public int GroupId { get; set; }
        public Group Group { get; set; } = null!;

        public int UserId { get; set; }
        public User User { get; set; } = null!;
    }

    public class AppContext : DbContext
    {
        public DbSet<User> Users { get; set; }
        public DbSet<Group> Groups { get; set; }
        public DbSet<GroupMember> GroupMembers { get; set; }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
            => optionsBuilder.LogTo(Console.WriteLine, LogLevel.Information)
            .EnableSensitiveDataLogging()
            .EnableDetailedErrors()
            .UseNpgsql("Host=localhost;Database=test;Username=guest;Password=pwd");

        protected override void OnModelCreating(ModelBuilder mb)
        {
            mb.Entity<Group>()
                .HasOne(x => x.GroupOwner)
                .WithMany()
                .HasForeignKey(x => new { x.Id, x.GroupOwnerId });

            mb.Entity<GroupMember>().HasKey(x => new { x.GroupId, x.UserId });

            mb.Entity<GroupMember>()
                .HasOne(x => x.User)
                .WithMany(x => x.Groups)
                .HasForeignKey(x => new { x.UserId });

            mb.Entity<GroupMember>()
                .HasOne(x => x.Group)
                .WithMany(x => x.Members)
                .HasForeignKey(x => new { x.GroupId });
        }
    }
}

Stack traces

CoreEventId.SaveChangesFailed[10000] (Microsoft.EntityFrameworkCore.Update) 
      An exception occurred in the database while saving changes for context type 'ef_many_to_many_updates.AppContext'.
      System.InvalidOperationException: The property 'Group.Id' is defined as read-only after it has been saved, but its value has been modified or marked as modified.
         at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntryBase.PrepareToSave()       
         at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.GetEntriesToSave(Boolean cascadeChanges)
         at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChanges(StateManager stateManager, Boolean acceptAllChangesOnSuccess)
         at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.<>c.<SaveChanges>b__114_0(DbContext _, ValueTuple`2 t)
         at Npgsql.EntityFrameworkCore.PostgreSQL.Storage.Internal.NpgsqlExecutionStrategy.Execute[TState,TResult](TState state, Func`3 operation, Func`3 verifySucceeded)
         at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChanges(Boolean acceptAllChangesOnSuccess)
         at Microsoft.EntityFrameworkCore.DbContext.SaveChanges(Boolean acceptAllChangesOnSuccess)        
Unhandled exception. System.InvalidOperationException: The property 'Group.Id' is defined as read-only after it has been saved, but its value has been modified or marked as modified.
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntryBase.PrepareToSave()
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.GetEntriesToSave(Boolean cascadeChanges)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChanges(StateManager stateManager, Boolean acceptAllChangesOnSuccess)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.<>c.<SaveChanges>b__114_0(DbContext _, ValueTuple`2 t)
   at Npgsql.EntityFrameworkCore.PostgreSQL.Storage.Internal.NpgsqlExecutionStrategy.Execute[TState,TResult](TState state, Func`3 operation, Func`3 verifySucceeded)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.SaveChanges(Boolean acceptAllChangesOnSuccess)
   at Microsoft.EntityFrameworkCore.DbContext.SaveChanges(Boolean acceptAllChangesOnSuccess)
   at Microsoft.EntityFrameworkCore.DbContext.SaveChanges()
   at ef_many_to_many_updates.Program.Main(String[] args) in C:\git\demos\ef-many-to-many-updates\Program.cs:line 31

EF Core version

10.0.0

Database provider

Npgsql.EntityFrameworkCore.PostgreSQL

Target framework

dotnet10

Operating system

win11

IDE

visual studio 2026