[英]How can I query the foreign key objects using Entity Framework?
I am trying to learn about Entity Framework 6, and I am running into an issue, that I have been able to reproduce in a test project:我正在尝试了解 Entity Framework 6,但遇到了一个问题,我已经能够在测试项目中重现该问题:
A Movie
has a Name
and a Revenue
. Movie
有Name
和Revenue
。 A Revenue
has a GrossIncome
: Revenue
有总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; }
}
I am trying to use EF6 code-first to persist some data about movies in the database:我正在尝试使用 EF6 代码优先在数据库中保留一些关于电影的数据:
public class MovieContext: DbContext
{
public MovieContext(): base("name=testDB") { }
public DbSet<Movie> Movies { get; set; }
public DbSet<Revenue> Revenues { get; set; }
}
I start by inserting a new movie, with its associated revenue in the DB:我首先插入一部新电影,并在数据库中插入其相关收入:
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();
}
I can see in SQL Server that the tables are created, and that a Movies.Revenue_Id
column has been created, with a foreign key relationship to Revenue.Id
.我可以在SQL Server看到创建表,以及一个Movies.Revenue_Id
列已创建,用一个外键关系Revenue.Id
。
If I try to query it using SQL, it works fine:如果我尝试使用 SQL 查询它,它工作正常:
SELECT Movies.Name, Revenues.GrossIncome
FROM Movies
LEFT JOIN Revenues ON Movies.Revenue_Id = Revenues.Id
returns返回
Name GrossIncome
----------------------
foo 10
However, if I try to use Entity Framework to query the data:但是,如果我尝试使用实体框架来查询数据:
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();
}
The console reads: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.
My question is simple: what am I doing wrong?我的问题很简单:我做错了什么? How are the foreign key values supposed to be read?应该如何读取外键值?
Just include the child entity you want to load:只需包含您要加载的子实体:
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();
}
This will load the movies - and also make sure that all the references to their respective .Revenue
references have been loaded, too.这将加载电影 - 并确保所有对各自.Revenue
引用的引用也已加载。
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