Using linq to calculate rating average in EF Core

Question

The user can search for companies by city, brand, service type and rating. I have developed such a query, but I am getting an error in the rating section. The error message is as follows:

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.

I encountered this error after adding the rating part. How can I fix this? I don't think it will be effective to pull the data in this way. How should I go about developing this place? I also share the codes I used for the query with you.

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;
        }

Thank you so much. Ramazan

Answer 1

You need to group by some Id like 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

Grouping in SQL has limitations - you can return only grouping key and aggregation result. For returning whole record, you have to join to original query gain.

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;
}