使用 linq 计算 EF Core 中的评分平均值

Question

用户可以按城市、品牌、服务类型和评级搜索公司。我开发了这样的查询，但在评级部分出现错误。报错信息如下：

The LINQ expression 'DbSet .Join( outer: DbSet, inner: f => f.Id, outerKeySelector: c => c.FirmId, innerKeySelector: (f, c) => new TransparentIdentifier<Firm, Comment>( outer = f, inner = c )) .GroupBy( source: ti => ti.Outer, keySelector: ti => new { firm = ti.Outer, rating = ti.Inner.Rate })' 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 either AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync(). See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.

我在添加评分部分后遇到了这个错误。我怎样才能解决这个问题？我认为通过这种方式拉取数据不会有效。我应该如何开发这个地方？我还与您分享我用于查询的代码。

public async Task<IEnumerable<Firm>> GetFirmsForCustomerSearch(int cityId, int brandId, int serviceTypeId, int rate)
        {
           var query = from firms in AracsalContext.Firm select firms;
            
            if (brandId > 0)
                query = from firms in query
                        join firmBrands in AracsalContext.Firmbrand on new { f1 = firms.Id, f2 = brandId } equals new { f1 = firmBrands.FirmId, f2 = firmBrands.BrandId }
                        select firms;

            if (serviceTypeId > 0)
                query = from firms in query
                        join firmServices in AracsalContext.Firmservice on new { f1 = firms.Id, f2 = serviceTypeId } equals new { f1 = firmServices.FirmId, f2 = firmServices.ServiceId }
                        select firms;

            if (cityId > 0)
                query = from firms in query
                        where firms.CityId == cityId
                        select firms;

            if (rate > 0)
            {
                query = from firms in query
                        join comments in AracsalContext.Comment on firms.Id equals comments.FirmId
                        group new
                        {
                            firm = firms,
                            rating = comments.Rate
                        } by firms into g
                        where g.Average(r => r.rating) > rate
                        select g.Key;
            }


            var result = await query.ToListAsync();
            return result;
        }

非常感谢。斋月

Answer 1

您需要按一些 ID 分组，例如 firms.Id

query = from firms in query
                        join comments in AracsalContext.Comment on firms.Id equals comments.FirmId
                        group new
                        {
                            firm = firms,
                            rating = comments.Rate
                        } by firms.Id into g
                        where g.Average(r => r.rating) > rate
                        select g.Key;

Answer 2

SQL 中的分组有限制 - 您可以 return 仅分组键和聚合结果。对于 return 整条记录，您必须加入原始查询增益。

if (rate > 0)
{
    filteredByRate = 
        from firms in query
        join comments in AracsalContext.Comment on firms.Id equals comments.FirmId
        group new
        {
            rating = comments.Rate
        } by new { firms.Id } into g
        where g.Average(r => r.rating) > rate
        select g.Key;

    query = 
        from films in query
        join f in filteredByRate on films.Id equals f.Id
        select films;
}

使用 linq 计算 EF Core 中的评分平均值

Using linq to calculate rating average in EF Core

c#

linq

performance

query-optimization

asp.net-core