Question

我有2个处于多对多关系的实体，我想将一个学生添加到班级中，但是从查询中收到班级对象后，该属性显示为空。我已经看到很多类似的问题，但是在这种情况下，没有答案真正帮助了我。

这是我的课程：

学生实体

class Student
{
    public virtual int Id { get; set; }
    public virtual string Name { get; set; }
    public virtual string Surname { get; set; }

    public Student()
    {
        Classes = new HashSet<Class>();
    }

    public virtual ICollection<Class> Classes { get; set; }
}

类实体

class Class
{
    public virtual int Id { get; set; }
    public virtual string ClassName { get; set; }

    public Class()
    {
        Students = new HashSet<Student>();
    }

    public virtual ICollection<Student> Students { get; set; }
}

上下文

class DatabaseContext : DbContext
{
    public DbSet<Student> Students { get; set; }
    public DbSet<Class> Classes { get; set; }

    public DatabaseContext()
    {
        Database.SetInitializer<DatabaseContext>(new Initializer());
    }
}

初始化器

class Initializer : DropCreateDatabaseAlways<DatabaseContext>
{
    protected override void Seed(DatabaseContext context)
    {
        Student student1 = new Student { Id = 1, Name = "Name", Surname = "Surname" };

        Class class1 = new Class { Id = 1, ClassName = "Math"};
        class1.Students.Add(student1); // The Count of the collection is 1

        context.Students.Add(student1);
        context.Classes.Add(class1);

        base.Seed(context);
    }
}

现在，当我尝试通过一种方法接收对象时，集合的计数为0

public static Class GetClass(int classId)
    {
        using (var context = new DatabaseContext())
        {
            Class receivedClass = context.Classes.Find(classId); // The collection is empty, the ClassName is there, though

            return receivedClass;
        }
    }

我想知道如何将一个对象添加到另一个对象的集合中，然后还可以使用该集合中的内容来检索该对象

Answer 1

我认为您正在寻找.Include()。 loading related entities有几种不同的策略。

using (var context = new DatabaseContext())
{
    // Load all students and related classes
    var classes1 = context.Classes
                    .Include(s => s.Students)
                    .ToList();

    // Load one student and its related classes
    var classt1 = context.Classes
                   .Where(s => s.Name == "someClassName")
                   .Include(s => s.Students)
                   .FirstOrDefault();

    // Load all students and related classes  
    // using a string to specify the relationship
    var classes2 = context.Classes
                    .Include("Students")
                    .ToList();

    // Load one student and its related classes  
    // using a string to specify the relationship
    var class2 = context.Classes
                   .Where(s => s.Name == "someName")
                   .Include("Students")
                   .FirstOrDefault();
}

实体框架返回没有值的ICollection <>

1 个答案: