在使用分组依据的 LINQ 查询中使用 DefaultIfEmpty() 时出错

Question

我有以下 LINQ (to SQL Server) 查询：

var railcarsByProduct = await (from r in DbContext.Railcars
                               let p = DbContext.ProductAliases
                                   .Where(pa => pa.Product.Company.CompanyCode == companyCode && pa.Alias == r.Product)
                                   .Select(pa => pa.Product.Name)
                                   .FirstOrDefault()
                               where r.Facility.Company.CompanyCode == companyCode && r.Departure == null
                               group r by p into productGroup
                               select new { Product = productGroup.Key, Count = productGroup.Count() }
                              ).ToListAsync();

这工作正常。 但是， ProductAliases子查询有可能返回 null。 在这种情况下，我想默认为r.Product 。

我尝试将DefaultIfEmpty()添加到子查询中。

var railcarsByProduct = await (from r in DbContext.Railcars
                               let p = DbContext.ProductAliases
                                   .Where(pa => pa.Product.Company.CompanyCode == companyCode && pa.Alias == r.Product)
                                   .Select(pa => pa.Product.Name)
                                   .DefaultIfEmpty(r.Product)
                                   .FirstOrDefault()
                               where r.Facility.Company.CompanyCode == companyCode && r.Departure == null
                               group r by p into productGroup
                               select new { Product = productGroup.Key, Count = productGroup.Count() }
                              ).ToListAsync();

但这给出了一个错误。

'LINQ 表达式 'DbSet() .Where(pa => pa.Product.Company.CompanyCode == __companyCode_0 && pa.Alias == r.Outer.Outer.Product) .Select(pa => pa.Product.Name) .DefaultIfEmpty(r.Outer.Outer.Product)' 无法翻译。 以可翻译的形式重写查询，或通过插入对“AsEnumerable”、“AsAsyncEnumerable”、“ToList”或“ToListAsync”的调用显式切换到客户端评估。

接下来，我尝试在group by子句中处理 null 情况。

var railcarsByProduct = await (from r in DbContext.Railcars
                               let p = DbContext.ProductAliases
                                   .Where(pa => pa.Product.Company.CompanyCode == companyCode && pa.Alias == r.Product)
                                   .Select(pa => pa.Product.Name)
                                   .FirstOrDefault()
                               where r.Facility.Company.CompanyCode == companyCode && r.Departure == null
                               group r by p ?? r.Product into productGroup
                               select new { Product = productGroup.Key, Count = productGroup.Count() }
                              ).ToListAsync();

但这也给出了完全相同的错误。

我知道可以只删除所有行，然后将它们分组到 C# 代码中。 但是有没有人看到我不需要这样做的方式？

Answer 1

尝试使用类似下面的东西。

var railcarsByProduct = await (from r in DbContext.Railcars
                               let p = DbContext.ProductAliases
                                   .Where(pa => pa.Product.Company.CompanyCode == companyCode && pa.Alias == r.Product)
                                   .Select(pa => pa.Product.Name)
                                   .FirstOrDefault() ?? r.Product
                               where r.Facility.Company.CompanyCode == companyCode && r.Departure == null
                               group r by p into productGroup
                               select new { Product = productGroup.Key, Count = productGroup.Count() }
                              ).ToListAsync();

不确定它是否会起作用，但我知道 DefaultIfEmpty 很奇怪，或者过去对实体框架很奇怪。

Answer 2

尝试以下查询：

var query = 
    from r in DbContext.Railcars
    from p in DbContext.ProductAliases
        .Where(pa => pa.Product.Company.CompanyCode == companyCode && pa.Alias == r.Product)
        .Select(pa => pa.Product.Name)
        .Take(1)
        .DefaultIfEmpty()
    where r.Facility.Company.CompanyCode == companyCode && r.Departure == null
    group r by p ?? r.Product into productGroup
    select new 
    { 
        Product = productGroup.Key, 
        Count = productGroup.Count() 
    };