在实体框架核心中包含集合 [英] Include collection in Entity Framework core

问题描述

例如，我有这个实体：

  public class Book 
 {
 [Key] 
 public string BookId {get;组; } 
 public List< BookPage>页面{get;组; } 
 public string Text {get;组; 
} 
 
 public class BookPage 
 {
 [Key] 
 public string BookPageId {get;组; } 
 public PageTitle PageTitle {get;组; } 
 public int Number {get;组; 
} 
 
 public class PageTitle 
 {
 [Key] 
 public string PageTitleId {get;组; } 
 public string标题{get;组; } 
} 
  

如果我只知道BookId，我应该如何加载所有的PageTitles？ p>

这是我如何尝试这样做：

 使用（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 
。选择（x => x.PageTitle）
 .ToList（）; 
} 
  

但问题是，它抛出异常

ArgumentException：属性表达式x => {从x中的页面y
中选择[y] .PageTitle}'无效。表达式应该代表
属性访问：'t => t.MyProperty'。当指定多个
属性时，使用匿名类型：'t => new {t.MyProperty1，
t.MyProperty2}'。参数名称：propertyAccessExpression

有什么问题，该怎么办？

解决方案

直接在 ThenInclude 中访问 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）
。选择（x => x.Pages）
 .Select（x => x.PageTitle）
 .ToList（）; 
} 
  

For example, I have this entities:

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; }
}

How should I load all PageTitles, if I Knew only BookId?

Here it is how I'm trying to do this:

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();
}

But the problem is, that it throws exception

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

What's wrong, what exactly should I do?

解决方案

Try accessing PageTitle directly in ThenInclude:

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();
}

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