泛型仓储中Linq分组查询翻译失败，如何避免客户端求值？

我实现了如下C#泛型仓储方法：

public async Task<IList<TGroupSelect>> GetAll<TEntity,TGroup,TGroupSelect>(Func<IQueryable<T>, IIncludableQueryable<T, object>> include = null,
           Expression<Func<T, bool>> whereClause = null, Expression<Func<T, TEntity>> selector = null,

          Func<IQueryable<TEntity>, IQueryable<IGrouping<object, TEntity>>> groupBy = null,
          Func<IGrouping<object, TEntity>, TGroupSelect> groupSelector = null)
{
    var result = _ctx.Set<T>().AsQueryable();

    result = include(result);

    var res = (whereClause is null ? result.Select(selector) : result.Where(whereClause).Select(selector));

    var group = groupBy(res);

    return  group.Select(groupSelector).ToList();
}

执行时抛出如下错误：

The LINQ expression 'DbSet<UserMeetings>()
    .Include(x => x.User)
    .Include(x => x.Meeting)
    .ThenInclude(x => x.Host)
    .Where(x => x.UserId == __UserId_0)
    .Select(x => x)
    .GroupBy(y => y.Meeting)' 

could not be translated. Either rewrite the query in a form that can
 be translated, or switch to client evaluation explicitly by inserting
 a call to 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or
 'ToListAsync'. 

调用示例代码：

var meetings =await  repo.GetAll<UserMeetings,Meeting, IGrouping<Meeting, UserMeetings>> (
                include:x=>x.Include(x=>x.User).Include(x=>x.Meeting).ThenInclude(x=>x.Host),
                whereClause: filter,
                selector: x=>x,
                groupBy: x => { return x.GroupBy(x => x.Meeting); }, 
                groupSelector:(x)=> x);

问题：如何在不调用AsAsyncEnumerable的前提下，让分组后的查询正常运行？

问题根源 #

EF Core无法将「按整个实体对象分组」的操作转换为SQL——SQL的GROUP BY仅支持基于标量值（如ID、字符串等）的分组，无法直接对复杂实体对象进行分组。当前代码按Meeting实体分组，超出了EF Core的SQL转换能力。

修改步骤 #

1. 调整仓储方法的泛型与委托类型
   将groupBy相关参数改为强类型表达式，让EF Core能正确解析分组逻辑：

   public async Task<IList<TGroupSelect>> GetAll<TEntity, TGroupKey, TGroupSelect>(
       Func<IQueryable<T>, IIncludableQueryable<T, object>> include = null,
       Expression<Func<T, bool>> whereClause = null,
       Expression<Func<T, TEntity>> selector = null,
       Expression<Func<TEntity, TGroupKey>> groupByKey = null,
       Func<IGrouping<TGroupKey, TEntity>, TGroupSelect> groupSelector = null)
   {
       var result = _ctx.Set<T>().AsQueryable();
   
       if (include != null)
           result = include(result);
   
       var res = whereClause == null ? result.Select(selector) : result.Where(whereClause).Select(selector);
   
       var grouped = res.GroupBy(groupByKey);
   
       return await grouped.Select(groupSelector).ToListAsync();
   }
   

2. 修改调用逻辑，按实体标量键分组
   调用时改为按Meeting的唯一标识（如MeetingId）分组，同时调整groupSelector获取所需数据结构：

   var meetings = await repo.GetAll<UserMeetings, int, (Meeting Meeting, List<UserMeetings> UserMeetings)>(
       include: x => x.Include(x => x.User).Include(x => x.Meeting).ThenInclude(x => x.Host),
       whereClause: filter,
       selector: x => x,
       groupByKey: x => x.MeetingId,
       groupSelector: g => (
           Meeting: g.First().Meeting,
           UserMeetings: g.ToList()
       ));
   

额外说明 #

• 若需要直接获取IGrouping结构，可将泛型参数TGroupSelect设为IGrouping<int, UserMeetings>，但分组键必须是标量值。
• 原代码中groupBy参数使用Func<IQueryable<TEntity>, ...>会导致EF Core无法解析内部表达式，改为Expression<Func<TEntity, TGroupKey>>才能让分组操作被转换为SQL。

内容的提问来源于stack exchange，提问作者PontiacGTX