c# - 在 Entity Framework Core 中包含集合

Question

例如，我有那些实体:

public class Book
{
    [Key]
    public string BookId { get; set; }
    public List<BookPage> Pages { get; set; }
    public string Text { get; set; }
} 

public class BookPage
{
    [Key]
    public string BookPageId { get; set; }
    public PageTitle PageTitle { get; set; }
    public int Number { get; set; }
}

public class PageTitle
{
    [Key]
    public string PageTitleId { get; set; }
    public string Title { get; set; }
}

如果我只知道 BookId，我应该如何加载所有的 PageTitles？

这是我尝试这样做的方式:

using (var dbContext = new BookContext())
{
    var bookPages = dbContext
        .Book
        .Include(x => x.Pages)
        .ThenInclude(x => x.Select(y => y.PageTitle))
        .SingleOrDefault(x => x.BookId == "some example id")
        .Pages
        .Select(x => x.PageTitle)
        .ToList();
}

但问题是，它会抛出异常

ArgumentException: The properties expression 'x => {from Pages y in x select [y].PageTitle}' is not valid. The expression should represent a property access: 't => t.MyProperty'. When specifying multiple properties use an anonymous type: 't => new { t.MyProperty1, t.MyProperty2 }'. Parameter name: propertyAccessExpression

怎么了，我应该怎么办？

Answer 1

尝试在 ThenInclude 中直接访问 PageTitle:

using (var dbContext = new BookContext())
{
    var bookPages = dbContext
    .Book
    .Include(x => x.Pages)
    .ThenInclude(y => y.PageTitle)
    .SingleOrDefault(x => x.BookId == "some example id")
    .Select(x => x.Pages)
    .Select(x => x.PageTitle)
    .ToList();
}