LINQ operation on an IQueryable that joins to another IQueryable that needs to be group byed?

Question

Consider three IQueryables: tableA, tableB, and tableC. They all have a composite key of 3 values, a, b, and c. I will need to get a sum amount of a column in tableB, d, partitioned by a, b, and c. How do I join the three tables together to get an accurate partition to select a new object? My compiler always shows an error.

here is my basic query that I have tried:

from A in tableA
join B in tableB on new { A.a, A.b, A.c } equals new { B.a, B.b, B.c } into p1
group B by new  { B.a, B.b, B.c } into p2

from q2 in p2
join C in tableC on new { A.a, A.b, A.c }  equals new { C.a, C.b, C.c }  into p3

from results in p3
select new NewObject
{
    Column1 = p3.A.a,
    Column2 = p3.A.b,
    Column3 = p3.A.c,
    Column4 = Sum(p3.C.d),
    Column5 = p3.B.e
};

I get this long error about ambiguous invocation and I get red lines all over the place. I guess I'm not too sure how to do multiple joins onto a group by'd IQueryable. Is that even possible?

Answer 1

I think in this scenario, your query could benefit by simplifying the joins, writing a select with all of the required columns, and then utilizing the fluent API as follows. I believe this query will achieve your desired result.

var model = (from A in tableA
             join B in tableB on new { A.a, A.b, A.c } equals new { B.a, B.b, B.c }
             join C in tableC on new { A.a, A.b, A.c }  equals new { C.a, C.b, C.c }
             select new {
                 Aa = A.a,
                 Ab = A.b,
                 Ac = A.c,
                 Ba = B.a,
                 Bb = B.b,
                 Bc = B.c,
                 Be = B.e,
                 Ca = C.a,
                 Cb = C.b,
                 Cc = C.c,
                 Cd = C.d
             })
             .GroupBy(x => new {x.Ba, x.Bb, x.Bc})
             .Select(grp => new {
                 Column1 = grp.First().Aa,
                 Column2 = grp.First().Ab,
                 Column3 = grp.First().Ac,
                 Column4 = grp.Sum(y => y.Cd),
                 Column5 = grp.First().Be
             }).ToList();