LINQ，无法创建 XXX 类型的常量值.此上下文中仅支持原始类型或枚举类型 [英] LINQ, Unable to create a constant value of type XXX. Only primitive types or enumeration types are supported in this context

问题描述

在我的应用程序中，我有讲师，他们有可以教授的课程列表，当我删除课程时，我想删除与讲师的连接.代码如下:

In my application I have Lecturers and they have list of Courses they can teach and when I'm deleting a course I want to remove connection to lecturers. Here's the code:

public void RemoveCourse(int courseId)
{
    using (var db = new AcademicTimetableDbContext())
    {
        var courseFromDb = db.Courses.Find(courseId);

        var toRemove = db.Lecturers
                        .Where(l => l.Courses.Contains(courseFromDb)).ToList();

        foreach (var lecturer in toRemove)
        {
            lecturer.Courses.Remove(courseFromDb);
        }

        db.SaveChanges();
    }
}

但它不起作用.我得到

NotSupportedException:无法创建 Course 类型的常量值.此上下文中仅支持原始类型或枚举类型.

NotSupportedException: Unable to create a constant value of type Course. Only primitive types or enumeration types are supported in this context.

我做错了什么?

推荐答案

您不能将 Contains 与非原始值一起使用.做

You can't use Contains with non-primitive values. Do

Where(l => l.Courses.Select(c => c.CourseId).Contains(courseId)

(或您使用的 Id 字段).

(or the Id field you use).

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