Products.Recommendations.cs

// map
from o in docs.Orders
from l in o.Lines
select new {
    l.Product,
    Related = o.Lines.Select(x=>x.Product)
                .Where(x=>x!=l.Product)
                .Select(x=>new { Count = 1, Product = x}),
    Count = 1
}

// reduce
from result in results
group result by result.Product into g
select new {
    Product = g.Key,
    Count = g.Sum(x=>x.Count),
    Related = g.SelectMany(x=>x.Related)
        .GroupBy(x=>x.Product)
        .Select(x=>new {
            Count = x.Sum(y=>y.Count),
            Product = x.Key
        })
        .OrderByDescending(x=>x.Count)
}