Question

这是我的课程

public class Activity
{
    public int ID {get;set;}
    public string Personnel { get; set; }
    public Location Location { get; set; }
}

public class Location
{
    [Key]
    public string Name { get; set; }
}

当我保存活动时，它会创建一个新的位置，即使它已经存在。如何设置它以使用现有的位置？

Answer 1

通过上下文加载现有的Location，并将其用作Activity

的属性值

例如：

using(var context = new MyDbContext())
{
   var location = context.Locations.FindByKey(/* set key*/);
   var activity = new Activity(){Personnel = "Foo", Location = location};
   context.Activities.Add(activity);
   context.SaveChanges();
}

Answer 2

将Key属性添加到Activity.ID，就像您对Location.Name

一样

public class Activity
{
    [Key]
    public int ID {get;set;}
    public string Personnel { get; set; }
    public Location Location { get; set; }
}

Answer 3

另一种方法是在添加活动之前将位置附加到上下文：

using(var context = new MyDbContext())
{
    var location = new Location { Name = "MyName" };
    context.Locations.Attach(location);

    var activity = new Activity { Personnel = "Foo", Location = location };
    context.Activities.Add(activity);
    context.SaveChanges();
}

它可以帮助您从数据库中获取位置。

另一个选项（需要更改模型）是将位置的外键公开给Activity类：

public class Activity
{
    public int ID {get;set;}
    public string Personnel { get; set; }
    [ForeignKey("Location")]
    public string LocationName { get; set; }
    public Location Location { get; set; }
}

然后您可以简单地指定FK值：

using(var context = new MyDbContext())
{
    var activity = new Activity { Personnel = "Foo", LocationName = "MyName" };
    context.Activities.Add(activity);
    context.SaveChanges();
}

在这种情况下保留Location导航属性null。