EF Core设置导航属性为null时触发IsLoaded相关异常求助

我尝试将导航属性设置为null，代码如下：

var entity = await _dbContext.Books
    .Include(x => x.Predecessor)
    .SingleOrDefaultAsync(x => x.Id == command.Book.Id, cancellationToken);

if (command.Book.PredecessorId != null)
{
    entity.Predecessor = _dbContext.Books.Find(command.Book.PredecessorId);
}
else
{
    entity.Predecessor = null;
}

await _dbContext.SaveChangesAsync(cancellationToken);

但出现如下错误：

System.InvalidOperationException : The navigation 'Book.Predecessor' cannot have 'IsLoaded' set to false because the referenced entity is non-null and is therefore loaded.

InternalEntityEntry.SetIsLoaded(INavigationBase navigation, Boolean loaded)
NavigationFixer.NavigationReferenceChanged(InternalEntityEntry entry, INavigationBase navigationBase, Object oldValue, Object newValue)
InternalEntityEntryNotifier.NavigationReferenceChanged(InternalEntityEntry entry, INavigation navigation, Object oldValue, Object newValue)
ChangeDetector.DetectNavigationChange(InternalEntityEntry entry, INavigationBase navigationBase)
ChangeDetector.LocalDetectChanges(InternalEntityEntry entry)
ChangeDetector.DetectChanges(IStateManager stateManager)
ChangeTracker.DetectChanges()
ChangeTracker.TryDetectChanges()
ChangeTracker.Entries()
CoreContext.OnBeforeSaving() Zeile 448

Book实体相关代码：

public partial class Book
{
    public Book()
    {
        Successors = new List<Book>();
    }

    public virtual ICollection<Book> Successors { get; set; }

    public virtual Book Predecessor { get; set; }

    public static void OnModelCreating(ModelBuilder modelBuilder)
    {
        OnModelCreating<Book>(modelBuilder);

        modelBuilder.Entity<Book>()
            .HasMany(p => p.Successors)
            .WithOne(q => q.Predecessor)
            .OnDelete(DeleteBehavior.ClientCascade);

        //...
    }
}

问题原因 #

你通过Include(x => x.Predecessor)预先加载了Predecessor导航属性，此时EF Core已将该导航属性标记为已加载（IsLoaded=true）。当你把entity.Predecessor设为null时，EF Core内部会尝试将该导航属性的IsLoaded改为false，但由于之前已经加载过非null的实体，这个操作违反了EF Core的状态跟踪规则，因此抛出错误。

解决方法 #

有两种可行的处理方式：

方式一：直接设置外键属性而非导航属性 #

如果Book实体有对应的外键字段（比如PredecessorId），直接修改外键值即可，EF Core会自动同步导航属性的状态，这种方式无需预先加载Predecessor，还能避免不必要的数据查询：

var entity = await _dbContext.Books
    .SingleOrDefaultAsync(x => x.Id == command.Book.Id, cancellationToken);

entity.PredecessorId = command.Book.PredecessorId;

await _dbContext.SaveChangesAsync(cancellationToken);

方式二：若必须操作导航属性，先标记为未加载 #

如果一定要通过导航属性修改，可以先调用Entry(entity).Reference(x => x.Predecessor).IsLoaded = false;，再设置为null：

var entity = await _dbContext.Books
    .Include(x => x.Predecessor)
    .SingleOrDefaultAsync(x => x.Id == command.Book.Id, cancellationToken);

if (command.Book.PredecessorId != null)
{
    entity.Predecessor = _dbContext.Books.Find(command.Book.PredecessorId);
}
else
{
    // 先标记导航属性为未加载
    _dbContext.Entry(entity).Reference(x => x.Predecessor).IsLoaded = false;
    entity.Predecessor = null;
}

await _dbContext.SaveChangesAsync(cancellationToken);

不过这种方式不如直接操作外键简洁，更推荐方式一。

内容的提问来源于stack exchange，提问作者DarkLeafyGreen