Question

我试图了解Entity Framework 6，但遇到一个问题，我已经能够在测试项目中重现该问题：

Movie有一个Name和一个Revenue。 Revenue具有GrossIncome：

public class Movie
{
        [Key]
        public int Id { get; set; }
        public string Name { get; set; }
        public Revenue Revenue { get; set; }
}

public class Revenue
{
        [Key]
        public int Id { get; set; }
        public double GrossIncome { get; set; }
}

我正在尝试先使用EF6代码将一些有关电影的数据保存在数据库中：

public class MovieContext: DbContext
{
    public MovieContext(): base("name=testDB") { }
    public DbSet<Movie> Movies { get; set; }
    public DbSet<Revenue> Revenues { get; set; }
}

我首先插入一部新电影，并将其相关收入收入数据库：

using (var context = new MovieContext())
{
    Revenue revenue = new Revenue()
                {
                    GrossIncome = 10
                };
    Movie movie = new Movie()
                {
                    Name = "foo",
                    Revenue = revenue
                };

    context.Movies.Add(movie);
    context.SaveChanges();
}

我可以在SQL Server中看到已经创建了表，并且已经创建了Movies.Revenue_Id列，并具有与Revenue.Id的外键关系。

如果我尝试使用SQL查询它，则效果很好：

SELECT Movies.Name, Revenues.GrossIncome
FROM Movies
LEFT JOIN Revenues ON Movies.Revenue_Id = Revenues.Id

返回

Name    GrossIncome
----------------------
foo         10

但是，如果我尝试使用Entity Framework查询数据：

using (var context = new MovieContext())
{
    List<Movie> movieList = context.Movies.ToList();
    Console.WriteLine("Movie Name: " + movieList[0].Name);

    if (movieList[0].Revenue == null)
    {
        Console.WriteLine("Revenue is null!");
    }
    else
    {
        Console.WriteLine(movieList[0].Revenue.GrossIncome);
    }

    Console.ReadLine();
}

控制台显示：

Movie Name: foo     <- It shows that the query works, and that the data in the main table is fetched.
Revenue is null!    <- Even though the data in the DB is correct, EF does not read the data from the foreign key.

我的问题很简单：我做错了什么？应该如何读取外键值？

Answer 1

只需包括您要加载的子实体：

S = c + d

这将加载电影-并且还确保已加载对它们各自的using (var context = new MovieContext()) { List<Movie> movieList = context.Movies .Include(m => m.Revenue) // ADD THIS INCLUDE .ToList(); Console.WriteLine("Movie Name: " + movieList[0].Name); if (movieList[0].Revenue == null) { Console.WriteLine("Revenue is null!"); } else { Console.WriteLine(movieList[0].Revenue.GrossIncome); } Console.ReadLine(); }引用的所有引用。

如何使用Entity Framework查询外键对象？

1 个答案: