如何在 Linq 中的 GroupBy 方法后 Select 新模型列表？

Question

我想要 select 分组行到新模型 list.this 是我的代码：

List<Model_Bulk> q = (from a in db.Advertises 
                      join c in db.Companies on a.AdvertiseCompanyID equals c.CompanyID
                      where a.AdvertiseActive == true 
                            && a.AdvertiseExpireDate.HasValue 
                            && a.AdvertiseExpireDate.Value > DateTime.Now 
                            && (a.AdvertiseObjectType == 1 
                                || a.AdvertiseObjectType == 2)
                      select c)
     .GroupBy(a => a.CompanyID).Select(a => new Model_Bulk
     {
         CompanyEmail = a.CompanyContactInfo.Email,
         CompanyID = a.CompanyID,
         CompanyName = a.CompanyName,
         Mobile = a.CompanyContactInfo.Cell,
         UserEmail = a.User1.Email,
         categories = a.ComapnyCategories
     }).ToList();

group by 后，我不能使用 Select，自然会出现此语法错误：

System.Linq.IGrouping' does not contain a definition for 'CompanyContactInfo' and no extension method 'CompanyContactInfo' accepting a first argument of type System.Linq.IGrouping' could be found (are you missing a using directive or an assembly reference?)

如果我尝试使用 SelectMany() method.but 结果将重复并且 groupby 方法无法正常工作：

List<Model_Bulk> q = (from a in db.Advertises
                      join c in db.Companies on a.AdvertiseCompanyID equals c.CompanyID
                      where a.AdvertiseActive == true 
                            && a.AdvertiseExpireDate.HasValue 
                            && a.AdvertiseExpireDate.Value > DateTime.Now 
                            && (a.AdvertiseObjectType == 1 
                                || a.AdvertiseObjectType == 2)
                      select c)
     .GroupBy(a => a.CompanyID).SelectMany(a => a).Select(a => new Model_Bulk
     {
         CompanyEmail = a.CompanyContactInfo.Email,
         CompanyID = a.CompanyID,
         CompanyName = a.CompanyName,
         Mobile = a.CompanyContactInfo.Cell,
         UserEmail = a.User1.Email,
         categories = a.ComapnyCategories
     }).ToList();

Answer 1

您可以使用 .Select(g => g.First()) 而不是 .SelectMany(a => a)。这将为您提供每组的第一项。

(from a in db.Advertises
                          join c in db.Companies on a.AdvertiseCompanyID equals c.CompanyID
                          where a.AdvertiseActive == true && a.AdvertiseExpireDate.HasValue && a.AdvertiseExpireDate.Value > DateTime.Now && (a.AdvertiseObjectType == 1 || a.AdvertiseObjectType == 2)
                          select c)
             .GroupBy(a => a.CompanyID)
             .Select(g => g.First())
             .Select(a => new Model_Bulk
             {
                 CompanyEmail = a.CompanyContactInfo.Email,
                 CompanyID = a.CompanyID,
                 CompanyName = a.CompanyName,
                 Mobile = a.CompanyContactInfo.Cell,
                 UserEmail = a.User1.Email,
                 categories = a.ComapnyCategories
             }).ToList();

请注意，这可能不受支持，如果是这种情况，请在 .Select(g => g.First())

之前添加一个 AsEnumerable 调用

Answer 2

您应该明白，在 LinQ 表达式中执行 GroupBy() 之后，您使用的是 group 所以在您的示例中这样写就好了:

List<Model_Bulk> q = 
    (from a in db.Advertises join c in db.Companies on a.AdvertiseCompanyID equals c.CompanyID                          
    where a.AdvertiseActive == true 
    && a.AdvertiseExpireDate.HasValue 
    && a.AdvertiseExpireDate.Value > DateTime.Now 
    && (a.AdvertiseObjectType == 1 || a.AdvertiseObjectType == 2)
    select c)
                 .GroupBy(a => a.CompanyID)
                 .Select(a => new Model_Bulk
                 {
                     CompanyEmail = a.First().CompanyContactInfo.Email,
                     CompanyID = a.Key, //Note this line, it's can be happened becouse of GroupBy()
                     CompanyName = a.First().CompanyName,
                     Mobile = a.First().CompanyContactInfo.Cell,
                     UserEmail = a.First().User1.Email,
                     categories = a.First().ComapnyCategories
                 }).ToList();

Answer 3

相反，您可以尝试这样的事情，而不是混合查询表达式和方法...（根据需要在 where / select 中使用 FirstOrDefault()）

(from a in db.Advertises
join c in db.Companies on a.AdvertiseCompanyID equals c.CompanyID
group a by new { a.CompanyId } into resultsSet
where resultsSet.AdvertiseActive == true && resultsSet.AdvertiseExpireDate.HasValue && resultsSet.AdvertiseExpireDate.Value > DateTime.Now && (resultsSet.AdvertiseObjectType == 1 || resultsSet.AdvertiseObjectType == 2)
select new Model_Bulk
{
    CompanyEmail = resultsSet.CompanyContactInfo.Email,
    CompanyID = resultsSet.CompanyID,
    CompanyName = resultsSet.CompanyName,
    Mobile = resultsSet.CompanyContactInfo.Cell,
    UserEmail = resultsSet.User1.Email,
    categories = resultsSet.ComapnyCategories
}).ToList();

如何在 Linq 中的 GroupBy 方法后 Select 新模型列表？

How to Select new model list after GroupBy mothod in Linq?

c#

linq

entity-framework

model

group-by