EF Core GroupBy with Select Distinct Count

Question

I have a table, Items , which has a many to one relationship with two distinct parents.

I want to select the counts of ParentA for each ParentB .

In SQL this is simple:

SELECT "ParentBId", count(distinct "ParentAId") 
FROM "Items"
GROUP BY "ParentBId"

In Linq I have this statement:

var itemCounts = await _context.Items
    .GroupBy(item => item.ParentBId,
        (parentBId, items) => new
        {
            ParentBId = parentBId,
            Count = items.Select(item => item.ParentAId).Distinct().Count(),
        }).ToDictionaryAsync(group => group.ParentBId, group => group.Count);

When running this query, EF is blowing up with this error:

System.InvalidOperationException: Processing of the LINQ expression 'AsQueryable<string>(Select<Item, string>(
    source: NavigationTreeExpression
        Value: default(IGrouping<string, Item>)
        Expression: (Unhandled parameter: e), 
    selector: (item) => item.ParentAId))' by 'NavigationExpandingExpressionVisitor' failed. This may indicate either a bug or a limitation in EF Core. See https://go.microsoft.com/fwlink/?linkid=2101433 for more detailed information.
   at Microsoft.EntityFrameworkCore.Query.Internal.NavigationExpandingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
   at Microsoft.EntityFrameworkCore.Query.Internal.NavigationExpandingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
...

The Items table does use Table per hierarchy with a discriminator column to determine what the item type is. I do not know if this is a factor.

I have seen lots of people recommend the items.Select(i => i.Field).Distinct().Count() option, but this doesn't seem to be working here. Any other suggestions?

Thanks!

Answer 1

Currently any kind of distinction inside groups (like Distinct inside ElementSelector of GroupBy or another GroupBy inside ElementSelector of GroupBy ) isn't supported by EF Core . If you insist on using EF in this case, you have to fetch some data in memory:

var result = (await _context.Items
              .Select(p => new { p.ParentAId, p.ParentBId })
              .Distinct()
              .ToListAsync())  // When EF supports mentioned cases above, you can remove this line!
              .GroupBy(i => i.ParentBId, i => i.ParentAId)
              .ToDictionary(g => g.Key, g => g.Distinct().Count());