并行循环中的LINQ到SQL:如何防止重复插入?

我在尝试并行化计算代价高昂的API集成时遇到了一些麻烦。

集成并行查询API并填充 ConcurrentBag 收藏。完成一些处理,然后将其传递给 Parallel.ForEach() 它通过linqtosql与数据库接口。

有:

1. 一个平行运行的外环

2. 通过纪律的内部循环

3. 在它里面,另一个循环遍历课程。

问题 我遇到的是:由于任何一节课可能属于多个课程,因此在课程上并行循环意味着有时一节课会被插入多次。

当前代码如下所示:

( externalCourseList 是类型的集合 ConcurrentBag<ExternalCourse> .)

Parallel.ForEach(externalCourseList, externalCourse =>
{
    using ( var context = new DataClassesDataContext() )
    {
        var dbCourse = context.Courses.Single( x => x.externalCourseId == externalCourse.courseCode.ToString() );

        dbCourse.ShortDesc = externalCourse.Summary;
        //dbCourse.LongDesc = externalCourse.MoreInfo;
        //(etc)

        foreach (var externalDiscipline in externalCourse.Disciplines)
        {
            var dbDiscipline = context.Disciplines.Where(
                x => x.ExternalDisciplineID == externalDiscipline.DisciplineCode.ToString() ).SingleOrDefault();

            if (dbDiscipline == null)
                dbDiscipline = new Linq2SQLEntities.Discipline();

            dbDiscipline.Title = externalDiscipline.Name;
            //(etc)
            dbDiscipline.ExternalDisciplineID = externalDiscipline.DisciplineCode.ToString();

            if (!dbDiscipline.IsLoaded)
                context.Disciplines.InsertOnSubmit(dbDiscipline);

            // relational table used as one-to-many relationship for legacy reasons
            var courseDiscipline = dbDiscipline.Course_Disciplines.SingleOrDefault(x => x.CourseID == dbCourse.CourseID);

            if (courseDiscipline == null)
            {
                courseDiscipline = new Course_Discipline
                {
                    Course = dbCourse,
                    Discipline = dbDiscipline
                };

                context.Course_Disciplines.InsertOnSubmit(courseDiscipline);
            }

            foreach (var externalLesson in externalDiscipline.Lessons)
            {
                /// The next statement throws an exception
                var dbLesson = context.Lessons.Where(
                    x => x.externalLessonID == externalLesson.LessonCode).SingleOrDefault();

                if (dbLesson == null)
                    dbLesson = new Linq2SQLEntities.Lesson();

                dbLesson.Title = externalLesson.Title;
                //(etc)
                dbLesson.externalLessonID = externalLesson.LessonCode;

                if (!dbLesson.IsLoaded)
                    context.Lessons.InsertOnSubmit(dbLesson);

                var disciplineLesson = dbLesson.Discipline_Lessons.SingleOrDefault(
                    x => x.DisciplineID == dbDiscipline.DisciplineID && x.LessonID == dbLesson.LessonID);

                if (disciplineLesson == null)
                {
                    disciplineLesson = new Discipline_Lesson
                    {
                        Discipline = dbDiscipline,
                        Lesson = dbLesson
                    };

                    context.Discipline_Lessons.InsertOnSubmit(disciplineLesson);
                }
            }

        }

        context.SubmitChanges();
    }
}
);

( IsLoaded 如所述实现 here .)

在前面的行中抛出异常 /// 因为同一节课经常被多次插入和调用 .SingleOrDefault() 在 context.Lessons.Where(x => x.externalLessonID == externalLesson.LessonCode) 失败。

解决这个问题的最好办法是什么?

谢谢。