如何让Entity Framework正确处理关联实体数组的修改？

我定义了带审计字段的抽象类AuditObject，用于在实体保存前自动设置字段：

public abstract class AuditObject
{
    public string CreatedBy { get; set; }
    public string UpdatedBy { get; set; }
}

在DbContext的保存前方法中设置审计字段：

private void OnBeforeSaving()
{
    var user = _someService.GetUserName();

    foreach (var entry in ChangeTracker.Entries()) 
    {
        if (entry.State == EntityState.Modified && entry.Entity is AuditObject auditObject) 
        {
            auditObject.UpdatedBy = user;
            entry.Property("CreatedBy").IsModified = false; 
        }
        else if (entry.State == EntityState.Added && entry.Entity is AuditObject auditObject)
        {
            auditObject.CreatedBy = user;
            auditObject.UpdatedBy = user;
        }
    }
}

继承该抽象类的实体及关联实体：

public abstract class Order: AuditObject
{
    public long Id { get; set; }
    public List<OrderProducts> Products { get; set; }
}

public abstract class OrderProducts: AuditObject
{
    public long Id { get; set; }
    public string Name { get; set; }
}

更新接口中直接替换Products列表：

[HttpPut("{id:long}")]
public async Task<Order> UpdateOrder(long id, Order order)
{
    var currentOrder = await _dbContext.Order
        .Include(x => x.Products)
        .FirstOrDefaultAsync(x => x.Id == order.Id);

    currentOrder.Products = order.Products;
    await _dbContext.SaveChangesAsync();

    return Ok(currentOrder);
}

调试时发现EF的ChangeTracker将此操作识别为“修改+删除”，创建新的Products记录并抛出异常：

System.InvalidOperationException: The property 'OrderProduct.CreatedBy' cannot be assigned a value generated by the database. Store-generated values can only be assigned to properties configured to use store-generated values.

仅在更新包含已有Product ID的列表时出现该异常，创建实体时无问题。需要解决的是：如何让EF替换Products列表时，不识别为修改+删除操作？

1. 手动同步关联实体状态（推荐） #

直接替换列表会让EF误判原有实体为删除、传入实体为新增。正确做法是对比新旧列表，手动处理增删改：

[HttpPut("{id:long}")]
public async Task<IActionResult> UpdateOrder(long id, Order order)
{
    var currentOrder = await _dbContext.Order
        .Include(x => x.Products)
        .FirstOrDefaultAsync(x => x.Id == order.Id);

    if (currentOrder == null)
    {
        return NotFound();
    }

    // 移除当前订单中不存在于传入列表的产品
    var productsToRemove = currentOrder.Products
        .Where(p => !order.Products.Any(op => op.Id == p.Id))
        .ToList();
    foreach (var product in productsToRemove)
    {
        _dbContext.OrderProducts.Remove(product);
    }

    // 处理新增或更新产品
    foreach (var incomingProduct in order.Products)
    {
        var existingProduct = currentOrder.Products.FirstOrDefault(p => p.Id == incomingProduct.Id);
        if (existingProduct == null)
        {
            currentOrder.Products.Add(incomingProduct);
        }
        else
        {
            // 复制需要更新的属性，CreatedBy会被OnBeforeSaving标记为未修改
            _dbContext.Entry(existingProduct).CurrentValues.SetValues(incomingProduct);
        }
    }

    await _dbContext.SaveChangesAsync();
    return Ok(currentOrder);
}

2. 手动设置传入实体的跟踪状态 #

如果坚持替换列表，需先将传入的已有实体附加到ChangeTracker并标记状态：

[HttpPut("{id:long}")]
public async Task<IActionResult> UpdateOrder(long id, Order order)
{
    var currentOrder = await _dbContext.Order
        .Include(x => x.Products)
        .FirstOrDefaultAsync(x => x.Id == order.Id);

    if (currentOrder == null)
    {
        return NotFound();
    }

    // 标记传入产品的状态
    foreach (var product in order.Products)
    {
        if (product.Id > 0)
        {
            // 已有实体标记为未变更，如需更新可改为Modified
            _dbContext.Entry(product).State = EntityState.Unchanged;
        }
        else
        {
            _dbContext.Entry(product).State = EntityState.Added;
        }
    }

    // 清空原有列表并添加传入列表
    currentOrder.Products.Clear();
    currentOrder.Products.AddRange(order.Products);

    await _dbContext.SaveChangesAsync();
    return Ok(currentOrder);
}

3. 检查CreatedBy字段配置 #

异常提示该字段被识别为数据库生成字段，需在实体配置中明确标记：

// 在DbContext的OnModelCreating方法中
modelBuilder.Entity<OrderProducts>()
    .Property(p => p.CreatedBy)
    .ValueGeneratedNever();

直接替换关联列表时，传入的OrderProducts实体不在当前ChangeTracker的跟踪范围内，EF会默认将其标记为Added状态，触发新增审计逻辑。同时原列表中的实体被标记为Deleted，两者叠加导致CreatedBy字段的处理冲突，最终抛出异常。

内容的提问来源于stack exchange，提问作者Matt