linq - 无法使 GroupJoin 工作。 NavigationExpandingExpressionVisitor 异常

Question

这是我第一次使用 GroupJoin。从我看到的示例来看，在其基本形式中使用似乎非常简单，但是当我使用它时，我总是会遇到 NavigationExpandingExpressionVisitor 异常。在这里，一个例子:

[Table("Users")]
public class WAUser
{
    public int Id { get; set; }
    
    //TODO: Unique key
    [NotNull]
    public string UserUuid { get; set; } 
    
    [DefaultValue(true)]
    public bool NotifyOnlineState { get; set; }
    [DefaultValue(true)]
    public bool NotifyOfflineState { get; set; }
}


public class WASubscription
{
    public int Id { get; set; }
    
    public string PackageIdentifier { get; set; } //Product package indentifier
    
    public DateTime? ExpiresAt { get; set; } //When the subscription or trial expires
    
    public bool Expired { get; set; }
    
    public bool IsTrial { get; set; }
    
    public int PhoneCount { get; set; } //Number of phones this subscriptions supplies
    
    public int UserId { get; set; }
    public WAUser User { get; set; }
}

var userSubscriptions = await dbContext.Users
                .GroupJoin(dbContext.Subscriptions,
                    u => u.Id,
                    s => s.UserId,
                    (u, subscriptions) => new
                    {
                        User = u,
                        Subscriptions = subscriptions
                    })
                .ToListAsync();

抛出的异常:

Unhandled exception. System.InvalidOperationException: Processing ofthe LINQ expression 'DbSet.GroupJoin(outer: DbSet,inner: u => u.Id,outerKeySelector: s => s.UserId,innerKeySelector: (u, subscriptions) => new {User = u,Subscriptions = subscriptions})' by 'NavigationExpandingExpressionVisitor' failed. This may indicate either a bug or a limitation in EF Core. Seehttps://go.microsoft.com/fwlink/?linkid=2101433 for more detailedinformation.

Answer 1

很高兴你问。
问题是 EF Core 团队没有(也不想)提供 GroupJoin翻译。请参阅我对此 EF Core GitHub 线程的评论 Query: Support GroupJoin when it is final query operator #19930并链接了我试图说服他们提供此类支持的讨论(例如，他们为支持 LINQ left outer join 模式所做的工作应该非常容易)。所以请去那里投票，否则争论是它“没有值(value)”。
话虽如此，在当前的 EF Core 中，要么添加和使用集合导航属性(最好)，要么使用相关子查询而不是 GroupJoin ，例如代替

.GroupJoin(dbContext.Subscriptions,
    u => u.Id,
    s => s.UserId,
    (u, subscriptions) => new
    {
        User = u,
        Subscriptions = subscriptions
    })

和

.Select(u => new
{
    User = u,
    Subscriptions = dbContext.Subscriptions.Where(s => u.Id == s.UserId) // <--
})