EF核心：IGrouping中的异步嵌套选择导致ArgumentException

Question

Updated with other relevant classes

public class TrendItem
{
    public string ItemTitle { get; set; }

    public IEnumerable<string> ItemValues { get; set; }
}

public class TrendValue
{
    public int Id { get; set; }

    public TrendResultType TrendType { get; set; }

    public string Value { get; set; }

    public Trend Trend { get; set; } // contains DateRecorded property
}

---

Please see the function below that leverages on EF Core (2.1):

    public async Task<List<TrendItem>> GetTrends(
        int companyId,
        TrendResultType type,
        DateTimeOffset startDate,
        DateTimeOffset endDate,
        RatingResultGroup group
    )
    {
        var data = _dataContext.TrendValues.Where(rr =>
                        rr.Trend.CompanyId == companyId &&
                        rr.TrendType == type &&
                        rr.Trend.DateRecorded >= startDate &&
                        rr.Trend.DateRecorded <= endDate);

        return await data.GroupBy(rr => new { rr.Trend.DateRecorded.Year, rr.Trend.DateRecorded.Month })
                         .Select(g => new TrendItem() { ItemTitle = $"{g.Key.Year}-{g.Key.Month}", ItemValues = g.Select(rr => rr.Value) })
                         .ToListAsync();
    }

I'm getting problems, specifically with the portion g.Select(rr => rr.Value) , where I intended to select a collection of values (strings).

Whenever I try to change that to something else like g.Sum(rr => int.Parse(rr.Value)) , it works fine. It's the retrieval of the collection that seems to be a problem.

I always get ArgumentException: Argument types do not match .

Is this due to the async function?

Answer 1

Hmm, I'd start by looking to simplify the data retrieval into an anonymous type to perform the grouping, then transform into the view model. It may be that EF is getting a bit confused working with the grouped items values.

It looks like you want to group by date (year-month) then have a list of the values in each group:

var query = _dataContext.TrendValues
              .Where(rr =>
                rr.Trend.CompanyId == companyId 
                && rr.TrendType == type 
                && rr.Trend.DateRecorded >= startDate 
                && rr.Trend.DateRecorded <= endDate)
              .Select(rr => new 
              { 
                TrendYear = rr.Trend.DateRecorded.Year,
                TrendMonth = rr.Trend.DateRecorded.Month,
                rr.Value
              }).ToListAsync();

var data = query.GroupBy(rr => new { rr.TrendYear, rr.TrendMonth })
            .Select(g => new TrendItem() { ItemTitle = $"{g.Key.Year}-{g.Key.Month}", ItemValues = g.ToList().Select(rr => rr.Value).ToList() })
            .ToList();
return data;

which could be simplified to:

var query = _dataContext.TrendValues
             .Where(rr =>
               rr.Trend.CompanyId == companyId &&
               rr.TrendType == type &&
               rr.Trend.DateRecorded >= startDate &&
               rr.Trend.DateRecorded <= endDate)
               .Select(rr => new 
               { 
                 TrendDate = rr.Trend.DateRecorded.Year + "-" + rr.Trend.DateRecorded.Month,
                 rr.Value
               }).ToListAsync();

var data = query.GroupBy(rr => rr.TrendDate)
             .Select(g => new TrendItem() { ItemTitle = $"{g.Key}", ItemValues = g.ToList().Select(rr => rr.Value).ToList() })
             .ToList();
return data;

The selecting of the TrendDate in the query may need some work if Trend.Year and Trend.Month are numeric fields.