Linq多对多查询困境求助：GroupJoin与SelectMany用法疑问

嘿，我懂你卡在多对多Linq查询的那种头疼感！先把你的实体类整理成清晰的格式，再给你几个常用的查询示例，应该能帮到你：

你的实体类定义 #

Course类 #

public class Course : BaseEntity {
    public Course() {
        CourseCollections = new HashSet<CourseCollection>();
    }
    public int Id { get; set; }
    public string Name { get; set; }
    public int CompanyId { get; set; }
    public virtual ICollection<CourseCollection> CourseCollections { get; set; }
}

CourseCollection类（根据多对多关联的常规结构补充完整） #

public class CourseCollection : BaseEntity {
    public int CourseId { get; set; }
    public virtual Course Course { get; set; }
    // 假设关联的集合类为Collection，你可以根据实际业务替换类名
    public int CollectionId { get; set; }
    public virtual Collection Collection { get; set; }
}

实用Linq查询示例 #

1. 查询指定课程关联的所有集合 #

var targetCourseId = 1;
var courseWithCollections = dbContext.Courses
    .Include(c => c.CourseCollections)
    .ThenInclude(cc => cc.Collection) // 关联查询集合信息
    .FirstOrDefault(c => c.Id == targetCourseId);

// 遍历输出关联的集合名称
foreach (var relation in courseWithCollections.CourseCollections)
{
    Console.WriteLine(relation.Collection.Name);
}

2. 查询指定集合下某公司的所有课程 #

var targetCollectionId = 2;
var targetCompanyId = 5;
var filteredCourses = dbContext.CourseCollections
    .Include(cc => cc.Course)
    .Where(cc => cc.CollectionId == targetCollectionId && cc.Course.CompanyId == targetCompanyId)
    .Select(cc => cc.Course)
    .ToList();

3. 查询未关联任何集合的课程 #

var coursesWithoutCollections = dbContext.Courses
    .Where(c => !c.CourseCollections.Any())
    .ToList();

如果你的实际业务需求和上面的示例不匹配，或者CourseCollection的结构和我补充的不一样，随时说细节，我再帮你调整！

内容的提问来源于stack exchange，提问作者Flimzy_Programmer