无法跟踪实体类型 'x' 的实例，因为已跟踪具有键值“{Id: 6}”的另一个实例

Question

我正在做一个 asp .netcore 6.0 干净架构项目。

当我尝试更新站点时，出现此错误，

System.InvalidOperationException: 无法跟踪实体类型 'SiteCode' 的实例，因为另一个具有键值 '{Id: 6}' 的实例已经被跟踪。附加现有实体时，确保只附加一个具有给定键值的实体实例。

在我们使用服务之前，同样的代码在那里运行良好。现在我们转向干净的架构（CQRS 和 Mediatr）。我使用了相同的更新代码，但出现此错误。

.AsNoTracking() 我试过 var result = await _DbContext.SiteCodes.FindAsync(request.Id).AsNoTracking();这一行，

但是出现错误，

'ValueTask<SiteCode?>' does not contain a definition for 'AsNoTracking' and no accessible extension method 'AsNoTracking' accepting a first argument of type 'ValueTask<SiteCode?>' could be found (are you missing a using directive or an assembly reference?) [Application]

这是我的代码

UpdateSiteCommandHandler.cs

public async Task<SiteCode> Handle(UpdateSiteCommand request, CancellationToken cancellationToken)
    {
        var result = _mapper.Map<SiteCode>(request);

        _DbContext.SiteCodes.Update(result);  // goes to exception after this line

        await _DbContext.SaveChangesAsync(cancellationToken);

        return result;
    }

GetSiteByIdQueryHandler.cs

public async Task<SiteCode> Handle(GetSiteByIdQuery request, CancellationToken cancellationToken)
    {
        
        var result = await _DbContext.SiteCodes.FindAsync(request.Id);

        if (result == null)
        {
            throw new NotFoundException(nameof(SiteCode), request.Id);
        }

        return result;

    }

控制器

public async Task<IActionResult> Update(int id, [FromBody] UpdateSiteCommand command)
        {
            command.Id = id;

            var siteCode = await _mediator.Send(new GetSiteByIdQuery(Id: id));

            var result = await _mediator.Send(command);
            
            return Ok(result);

        }

DependencyInjection.cs

public static class DependencyInjection
{
    public static IServiceCollection AddApplication(this IServiceCollection services)
    {
        services.AddAutoMapper(Assembly.GetExecutingAssembly());
        services.AddValidatorsFromAssembly(Assembly.GetExecutingAssembly());
        services.AddMediatR(Assembly.GetExecutingAssembly());
        services.AddTransient(typeof(IPipelineBehavior<,>), typeof(PerformanceBehaviour<,>));  // I tried with AddScoped() But not work
       
        return services;
    }

}

在 DbContext 中

public override async Task<int> SaveChangesAsync(CancellationToken cancellationToken = default(CancellationToken))
        {
            var entries = ChangeTracker
                .Entries()
                .Where(e => e.Entity is AuditableEntity && (
                        e.State == EntityState.Added
                        || e.State == EntityState.Modified));

            foreach (var entityEntry in entries)
            {
                if (entityEntry.State == EntityState.Added)
                {
                    ((AuditableEntity)entityEntry.Entity).CreatedAt = DateTime.UtcNow;
                    ((AuditableEntity)entityEntry.Entity).CreatedBy = _httpContextAccessor?.HttpContext?.User?.Identity?.Name ?? null;
                }
                else
                {
                    Entry((AuditableEntity)entityEntry.Entity).Property(p => p.CreatedAt).IsModified = false;
                    Entry((AuditableEntity)entityEntry.Entity).Property(p => p.CreatedBy).IsModified = false;
                }

                ((AuditableEntity)entityEntry.Entity).ModifiedAt = DateTime.UtcNow;
                ((AuditableEntity)entityEntry.Entity).ModifiedBy = _httpContextAccessor?.HttpContext?.User?.Identity?.Name ?? null;
            }

            var result = await base.SaveChangesAsync(cancellationToken)

         return result;
        }

有人知道如何解决这个问题吗？

Answer 1

尝试在 Handler (GetSiteByIdQueryHandler.cs) 中将 SiteCodes 之后的方法 AsNoTracking() 添加到您的查询中。

public async Task<SiteCode> Handle(GetSiteByIdQuery request, CancellationToken cancellationToken)
{
    
    var result = await _DbContext.SiteCodes.AsNoTracking().FindAsync(request.Id);

    if (result == null)
    {
        throw new NotFoundException(nameof(SiteCode), request.Id);
    }

    return result;
}

Answer 2

EF Core 使用 ChangeTracker 检测加载实体的变化，更好（不是更快）的解决方案是加载实体进行更新。

public async Task<SiteCode> Handle(UpdateSiteCommand request, CancellationToken cancellationToken)
{
    // the following code will load entitiy if it is still not loaded.
    var dbRequest = await _DbContext.SiteCodes.FindAsync(request.Id);

    if (dbRequest == null)
        throw new Exception("Not found");

    _mapper.Map<SiteCode>(request, dbRequest);

    await _DbContext.SaveChangesAsync(cancellationToken);

    return result;
}