如何按嵌套Enumerable属性使用GroupBy分组Activity数据

数据结构 #

public class Activity
{
    public int Id { get; set; }
    public string Name { get; set; }
    public IEnumerable<Business> Businesses { get; set; }
}

public class Business
{
    public int Id { get; set; }
    public string Name { get; set; }
}

现有数据 #

• Activity 1
  • Business A
  • Business B
• Activity 2
  • Business B
• Activity 3
  • Business A

期望结果 #

• Business A
  • Activity 1
  • Activity 3
• Business B
  • Activity 1
  • Activity 2

问题 #

尝试了GroupBy与SelectMany/Select的多种组合，但始终无法正确实现分组映射。

解决方案 #

核心思路是先将Activity与对应的Business展开为一对多的关联项，再按Business分组，最后整理结构：

代码实现 #

// 假设activities是你的IEnumerable<Activity>数据源
var groupedResult = activities
    // 展开每个Activity和它的Business，生成(Activity, Business)的序列
    .SelectMany(activity => activity.Businesses.Select(business => new { activity, business }))
    // 按Business分组（如果Business的相等性判断有问题，建议按Id分组）
    .GroupBy(item => item.business)
    // 整理成期望的结构：每个组包含Business和对应的Activity列表
    .Select(group => new 
    {
        Business = group.Key,
        Activities = group.Select(item => item.activity).ToList()
    })
    .ToList();

注意事项 #

如果直接按Business对象分组时出现分组异常（比如同一个Business被分成多个组），说明Business类的相等性判断逻辑不符合预期，此时可以改为按Business.Id（唯一标识）分组，调整后的代码如下：

var groupedResult = activities
    .SelectMany(activity => activity.Businesses.Select(business => new { activity, business.Id, business.Name }))
    .GroupBy(item => new { item.Id, item.Name }) // 按Business的Id和Name分组，确保唯一性
    .Select(group => new 
    {
        Business = new Business { Id = group.Key.Id, Name = group.Key.Name },
        Activities = group.Select(item => item.activity).ToList()
    })
    .ToList();

内容的提问来源于stack exchange，提问作者trh88