[Bug]: CommandBatchPreparer.AddUniqueValueEdges fails to order UPDATE commands on unique index (Regression in 11.0 Preview 7)

[bkoelman]

Bug description

When reassigning a 1:1 relationship with a unique index (foreign key on dependent side using a shadow property) to an attached entity, DbContext.SaveChangesAsync() throws a unique constraint violation exception (duplicate key value violates unique constraint "IX_...").

This worked as expected in EF Core 9, EF Core 10, and EF Core 11.0.0-preview.6, but fails in EF Core 11.0.0-preview.7.

This regression was introduced by PR #38581 (commit fb2b999e9b6acc578cff47edec132044618b6028).

This occurs across all relational providers: PostgreSQL (Npgsql), SQL Server (Microsoft.EntityFrameworkCore.SqlServer), and SQLite (Microsoft.EntityFrameworkCore.Sqlite).

Root Cause Analysis

PR #38581 ("Avoid false circular dependency edges for unchanged unique index values during update batching", fixing #35588) added checks to CommandBatchPreparer.AddUniqueValueEdges to skip edge creation for Modified entities when the unique index value is unchanged:

// Pass 2: Adding predecessor edges to successor commands
if (command.EntityState == EntityState.Modified)
{
    var (originalValue, _) = rowIndexValueFactory.CreateEquatableIndexValue(command, fromOriginalValues: true);
    if (Equals(originalValue, value))
    {
        continue;
    }
}

However, when an untracked entity c0 is attached via DbContext.Attach(c0) and then assigned to a principal g1.Color = c0:

1. c0's shadow foreign key / unique index column GroupId is updated to G1 during navigation fixup, with IsModified = true and IsWrite = true. Because c0 was attached, both its OriginalValue and CurrentValue evaluate to G1.
2. The entity previously associated with G1 (c1) has GroupId cleared (OriginalValue = G1, CurrentValue = NULL).
3. In CommandBatchPreparer.AddUniqueValueEdges:
   • Pass 1 (Predecessors): c1 (OriginalValue = G1, CurrentValue = NULL) is registered in indexPredecessorsMap[G1] = [c1].
   • Pass 2 (Successors): When evaluating c0 (CurrentValue = G1), EF Core should create a dependency edge c1 -> c0 so c1 releases GroupId before c0 claims it.
   • However, because c0 has originalValue (G1) == value (G1), the new check in PR #38581 executes continue; and skips adding the dependency edge c1 -> c0.
4. Without the dependency edge in _modificationCommandGraph, c0 is executed before c1 (UPDATE RgbColors SET GroupId = 'G1' WHERE Id = '0x9a79e4'), violating the unique constraint IX_RgbColors_GroupId.

Your code

using System;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.EntityFrameworkCore;

// Minimal runnable reproduction
// Packages:
//   - Microsoft.EntityFrameworkCore (11.0.0-preview.7.*)
//   - Microsoft.EntityFrameworkCore.SqlServer (11.0.0-preview.7.*) OR
//   - Microsoft.EntityFrameworkCore.Sqlite (11.0.0-preview.7.*) OR
//   - Npgsql.EntityFrameworkCore.PostgreSQL (11.0.0-preview.6/preview.7)

var provider = Provider.Sqlite; // Change to Provider.SqlServer or Provider.Postgres to test other databases

await using var dbContext = new AppDbContext(provider);
await dbContext.Database.EnsureDeletedAsync();
await dbContext.Database.EnsureCreatedAsync();

// 1. Seed: Group0 has Color0, Group1 has Color1
var group0 = new WorkItemGroup { Name = "G0" };
var group1 = new WorkItemGroup { Name = "G1" };
var color0 = new RgbColor { Id = "0x9a79e4", DisplayName = "red" };
var color1 = new RgbColor { Id = "0xb4e6b9", DisplayName = "turquoise" };

group0.Color = color0;
group1.Color = color1;
dbContext.Groups.AddRange(group0, group1);
await dbContext.SaveChangesAsync();

// 2. In a new context: reassign Group1's color to Color0
await using var updateContext = new AppDbContext(provider);
var g1 = await updateContext.Groups.Include(g => g.Color).FirstAsync(g => g.Name == "G1");
var c0 = new RgbColor { Id = "0x9a79e4" };
updateContext.Attach(c0);
g1.Color = c0;

// In EF Core 10 and EF Core 11.0.0-preview.6:
// Color1 (SET GroupId = NULL) executes BEFORE Color0 (SET GroupId = G1) -> Succeeds!
//
// In EF Core 11.0.0-preview.7:
// Color0 (SET GroupId = G1) executes BEFORE Color1 (SET GroupId = NULL) -> Throws DbUpdateException
await updateContext.SaveChangesAsync();

Console.WriteLine("Saved successfully!");

public class WorkItemGroup
{
    public Guid Id { get; set; } = Guid.NewGuid();
    public string Name { get; set; } = "";
    public RgbColor? Color { get; set; }
}

public class RgbColor
{
    public string Id { get; set; } = "";
    public string? DisplayName { get; set; }
    public WorkItemGroup? Group { get; set; }
}

public enum Provider
{
    Postgres,
    SqlServer,
    Sqlite
}

public class AppDbContext : DbContext
{
    private readonly Provider _provider;

    public AppDbContext(Provider provider)
    {
        _provider = provider;
    }

    public DbSet<WorkItemGroup> Groups => Set<WorkItemGroup>();
    public DbSet<RgbColor> RgbColors => Set<RgbColor>();

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        switch (_provider)
        {
            case Provider.Sqlite:
                optionsBuilder.UseSqlite("Data Source=test_efcore_repro.db");
                break;
            case Provider.SqlServer:
                optionsBuilder.UseSqlServer("Server=localhost,1433;Database=test_efcore_repro;User Id=sa;Password=Passw0rd!;TrustServerCertificate=True");
                break;
            case Provider.Postgres:
                optionsBuilder.UseNpgsql("Host=localhost;Port=5432;Database=test_efcore_repro;Username=postgres;Password=postgres");
                break;
        }
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<WorkItemGroup>()
            .HasOne(g => g.Color)
            .WithOne(c => c.Group)
            .HasForeignKey<RgbColor>("GroupId");
    }
}

Stack traces

PostgreSQL (Npgsql):

Microsoft.EntityFrameworkCore.DbUpdateException: An error occurred while saving the entity changes. See the inner exception for details.
 ---> Npgsql.PostgresException (0x80004005): 23505: duplicate key value violates unique constraint "IX_RgbColors_GroupId"
DETAIL: Key ("GroupId")=(01a06c7d-c9bf-7a22-af83-dac4c89d178f) already exists.
   at Microsoft.EntityFrameworkCore.Update.ReaderModificationCommandBatch.ExecuteAsync(IRelationalConnection connection, CancellationToken cancellationToken)

SQL Server (Microsoft.EntityFrameworkCore.SqlServer):

Microsoft.EntityFrameworkCore.DbUpdateException: An error occurred while saving the entity changes. See the inner exception for details.
 ---> Microsoft.Data.SqlClient.SqlException (0x80131904): Cannot insert duplicate key row in object 'dbo.RgbColors' with unique index 'IX_RgbColors_GroupId'. The duplicate key value is (c5636c6b-c4ad-4297-94bf-fbbc681dff64).
   at Microsoft.EntityFrameworkCore.Update.ReaderModificationCommandBatch.ExecuteAsync(IRelationalConnection connection, CancellationToken cancellationToken)

SQLite (Microsoft.EntityFrameworkCore.Sqlite):

Microsoft.EntityFrameworkCore.DbUpdateException: An error occurred while saving the entity changes. See the inner exception for details.
 ---> Microsoft.Data.Sqlite.SqliteException (0x80004005): SQLite Error 19: 'UNIQUE constraint failed: RgbColors.GroupId'.
   at Microsoft.EntityFrameworkCore.Update.ReaderModificationCommandBatch.ExecuteAsync(IRelationalConnection connection, CancellationToken cancellationToken)

EF Core version

11.0.0-preview.7.* (worked in 11.0.0-preview.6, 10.0.x, and 9.0.x)

Database provider

Microsoft.EntityFrameworkCore.SqlServer / Microsoft.EntityFrameworkCore.Sqlite / Npgsql.EntityFrameworkCore.PostgreSQL

Target framework

.NET 11.0

Operating system

Windows 11

IDE

Visual Studio 2026 / VS Code / Rider