c# - "Include"在 Entity Framework Core 中 SelectMany + Select 后不工作

Question

我使用 Entity Framework Core (v2) 进行此查询，但 Include/ThenInclude 没有按我预期的那样工作。这是查询:

 var titlesOwnedByUser = context.Users
                   .Where(u => u.UserId == userId)
                   .SelectMany(u => u.OwnedBooks)
                   .Select(b => b.TitleInformation)
                   .Include(ti => ti.Title)
                   .ThenInclude(n => n.Translations);

查询有效，但我获得的标题的标题设置为 null。

只是为了澄清这些类

class User 
{
     public int Id { get; set; }
     public List<BookUser> OwnedBooks { get; set; }
}

class Book 
{
    public int Id { get; set; }
    public TitleInformation TitleInformation { get; set; }
    public List<BookUser> Owners { get; set; }
}

class BookUser 
{
     public int BookId { get; set; }
     public int UserId { get; set; }
     public Book Book { get; set; }
     public User User { get; set; }
}

class MyContext
{
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
        modelBuilder.Entity<BookUser>()
            .HasOne(x => x.User)
            .WithMany(x => x.OwnedBooks)
            .HasForeignKey(x => x.UserId);

        modelBuilder.Entity<BookUser>()
            .HasOne(x => x.Book)
            .WithMany(x => x.Owners)
            .HasForeignKey(x => x.BookId);
     }
}

class TitleInformation
{
    public int Id { get; set; }
    public Title Title { get; set; }
    public Title Subtitle { get; set; }
}

class Title
{
     public int Id { get; set; }
     public string OriginalTitle { get; set; }
     public List<Translation> Translations { get; set; }
}

我需要做什么才能使翻译在返回的可查询中加载？

Answer 1

这是 Loading Related Data - Ignored includes 中描述的当前 EF Core 限制:

If you change the query so that it no longer returns instances of the entity type that the query began with, then the include operators are ignored.

据此，你需要从context.Set<TitleInformation>()开始查询.但是为了产生所需的过滤，您需要来自 TitleInformation 的反向导航属性。至 Book您的模型中目前缺少:

class TitleInformation
{
    // ...
    public Book Book { get; set; } // add this and map it properly with fluent API
}

一旦你有了它，你就可以使用这样的东西:

var titlesOwnedByUser = context.Set<TitleInformation>()
    .Include(ti => ti.Title)
        .ThenInclude(n => n.Translations)
    .Where(ti => ti.Book.Owners.Any(bu => bu.UserId == userId));

或者，如果TitleInformation之间的关系和 Book是一对多的(上面是一对一的):

class TitleInformation
{
    // ...
    public List<Book> Books { get; set; }
}

分别是:

var titlesOwnedByUser = context.Set<TitleInformation>()
    .Include(ti => ti.Title)
        .ThenInclude(n => n.Translations)
    .Where(ti => ti.Books.SelectMany(b => b.Owners).Any(bu => bu.UserId == userId));