EF 有时会在外键类型上返回 null

Question

我有一个 WebAPI 控制器需要从数据库中返回一个集合，但我还需要从一些实体中删除一些属性。但是，由于某种原因，当我在调试模式下单步执行代码时，表示另一个实体的属性会返回 null，但 sometimes 除外。我在下面显示了将 Sport、HomeTeam 和 AwayTeam 显示为 null 的 json 响应：

但是当我在调试模式下单步执行代码时，我得到了预期的结果：

相关控制器方法如下

public dynamic Get()
{
    var allPicks = _db.Picks;
    if (User.Identity.IsAuthenticated)
    {
        string userId = User.Identity.GetUserId();
        var picks = _db.UnlockedPicks
            .Include(p => p.SelectedPick.AwayTeam)
            .Include(p => p.SelectedPick.HomeTeam)
            .Include(p => p.SelectedPick.Sport)
            .Where(p => p.UserId == userId);

        var unlockedPicks = picks.Select(p => p.SelectedPick);
        var otherPicks = allPicks.Except(unlockedPicks).Select(p => new
        {
            Analysis = "",
            PickSummary = "",
            Title = p.Title,
            Id = p.Id,
            Sport = p.Sport,
            HomeTeam = p.HomeTeam,
            AwayTeam = p.AwayTeam,
            MatchTime = p.MatchTime,
            PublishTime = p.PublishTime
        });
        return new
        {
            UnlockedPicks = unlockedPicks.OrderByDescending(p => p.MatchTime),
            OtherPicks = otherPicks.OrderByDescending(p => p.MatchTime)
        };
    }
    var publicPicks = allPicks.OrderByDescending(p => p.MatchTime).Select(p => new
    {
        Analysis = "",
        PickSummary = "",
        Title = p.Title,
        Id = p.Id,
        Sport = p.Sport,
        HomeTeam = p.HomeTeam,
        AwayTeam = p.AwayTeam,
        MatchTime = p.MatchTime,
        PublishTime = p.PublishTime
    });
    return new { UnlockedPicks = new Pick[0], OtherPicks = publicPicks };
}

没有Include()，它每次都返回null，但我怎样才能让它每次都返回预期的结果？

Answer 1

这是因为您没有使用延迟加载。禁用延迟加载时，您必须调用 Include 才能加载导航属性。如果要加载所有导航属性，而不调用Include，则必须激活延迟加载，这可以通过上下文的构造函数完成。像这样：

public partial class SchoolDBEntities : DbContext
{
    public SchoolDBEntities(): base("name=SchoolDBEntities")
    {
        this.Configuration.LazyLoadingEnabled = true;
    }
}

默认情况下应该启用延迟加载。您可能在代码中的其他地方禁用了它。每次在调试模式下检查导航属性时也会自动调用它。