与 ADO.NET 实体数据模型的简单关系

Question

我刚刚在 C# 中启动了一个简单的项目并添加了“ADO.NET 实体数据模型”

我的问题很简单，我有一个“课程”表和一个“用户”表，以及 user_id 和 course.user_id 之间的外键。我确实按照this msdn tutorial 实现了表模型，但它没有说明两个表之间的关系。

这是我的失败代码：

User user = null;
string title = "title";
string content = "content";

using (var db = new Entities())
{
    //search for the last user
    var query = from b in db.Users
                orderby b.RegisterDate descending
                select b;
    foreach (var item in query.Take(1))
    {
        user = item;
    }

    //create lesson
    Lesson lesson = new Lesson
    {
        Content = content,
        Grade = "X1",
        PostDate = DateTime.Now,
        Title = title,
        User = user, // user {System.Data.Entity.DynamicProxier.User_1DBAF22....}
        UserId = user.ID
    };
    db.Lessons.Add(lesson);
    db.SaveChanges(); //DBUpdateException was unhandled \n Unable to update the EntitySet 'Lessons' because it has a DefiningQuery and no <InsertFunction> element exists in the <ModificationFunctionMapping> element to support the current operation.
}

Answer 1

为您的数据库表定义一个主键。