为何无法将值对象ActivitySettings的PastDaysAllowedThreshold设为0？

我为ApplicationSettings实体配置了如下Fluent API：

builder.OwnsOne(
    applicationSettings => applicationSettings.ActivitySettings,
    builder =>
    {
        builder.Property(activitySettings => activitySettings.PastDaysAllowedThreshold)
            .HasColumnName("ActivityPastDaysAllowedThreshold")
            .HasDefaultValue(7);

        builder.HasCheckConstraint(
            "CK_ApplicationSettings_ActivityPastDaysAllowedThreshold",
            "ActivityPastDaysAllowedThreshold >= 0");
    });

ActivitySettings类定义如下：

public sealed class ActivitySettings // : ValueObject
{
    public ActivitySettings(int pastDaysAllowedThreshold)
    {
        EnsureArg.IsGte(pastDaysAllowedThreshold, 0, nameof(pastDaysAllowedThreshold));

        PastDaysAllowedThreshold = pastDaysAllowedThreshold;
    }

    public int PastDaysAllowedThreshold { get; } // already tried adding a setter.
}

ApplicationSettings包含以下方法：

public void SetActivitySettings(ActivitySettings activitySettings)
{
    EnsureArg.IsNotNull(activitySettings, nameof(activitySettings));

    ActivitySettings = activitySettings;

    RaiseDomainEvent(new ActivitySettingsUpdated(Id, activitySettings));
}

所有功能均正常，但将PastDaysAllowedThreshold设为0时，仓储跟踪的实体显示值已设为0，但数据库中并未更新，请问这是什么原因？

原因分析及解决方案 #

1. 缺少无参数构造函数导致EF Core变更检测异常 #

EF Core在加载实体或进行变更检测时，需要能够实例化ActivitySettings对象。当前你的ActivitySettings只有带参数的公开构造函数，EF Core可能无法正确初始化该对象以跟踪原始值，导致即使内存中值已变更，EF Core也识别不到需要更新数据库。

解决方法：添加一个私有无参数构造函数供EF Core使用：

public sealed class ActivitySettings // : ValueObject
{
    // 供EF Core使用的私有无参构造函数
    private ActivitySettings() { }

    public ActivitySettings(int pastDaysAllowedThreshold)
    {
        EnsureArg.IsGte(pastDaysAllowedThreshold, 0, nameof(pastDaysAllowedThreshold));
        PastDaysAllowedThreshold = pastDaysAllowedThreshold;
    }

    public int PastDaysAllowedThreshold { get; }
}

2. 值对象未重写Equals/GetHashCode导致变更判断错误 #

ActivitySettings作为值对象，应该基于属性值判断相等性，而非引用相等。当前EF Core会用引用比较来判断ActivitySettings是否变更，规范上值对象需要重写相等性方法，避免EF Core误判变更状态。

解决方法：重写Equals和GetHashCode方法：

public sealed class ActivitySettings // : ValueObject
{
    private ActivitySettings() { }

    public ActivitySettings(int pastDaysAllowedThreshold)
    {
        EnsureArg.IsGte(pastDaysAllowedThreshold, 0, nameof(pastDaysAllowedThreshold));
        PastDaysAllowedThreshold = pastDaysAllowedThreshold;
    }

    public int PastDaysAllowedThreshold { get; }

    public override bool Equals(object? obj)
    {
        if (obj is not ActivitySettings other)
            return false;

        return PastDaysAllowedThreshold == other.PastDaysAllowedThreshold;
    }

    public override int GetHashCode()
    {
        return PastDaysAllowedThreshold.GetHashCode();
    }
}

3. 验证EF Core生成的SQL语句 #

可以启用EF Core的日志功能，查看生成的SQL语句是否包含对ActivityPastDaysAllowedThreshold字段的更新，确认问题出在EF Core的变更检测还是数据库层面。

配置日志方法：在DbContext的OnConfiguring中添加日志输出：

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
    optionsBuilder.LogTo(Console.WriteLine, Microsoft.Extensions.Logging.LogLevel.Information);
}

运行代码后查看控制台输出的SQL，若更新语句中没有包含该字段，说明EF Core未识别到变更；若包含但数据库未更新，需检查数据库约束或事务是否提交。

内容的提问来源于stack exchange，提问作者Bassie