>>I got as far as
>>
>>

>> var invoicesList = (from invoices in _dbSet.Where(x => x.Account == accountName && x.Hidden == false)
>>                                join transact in _siriusContext.Transact on invoices.InvoiceNo equals transact.InvoiceNo into InvoiceBalance
>>                                from rt in InvoiceBalance.DefaultIfEmpty()
>>                                select new InvoicesList
>>                                {
>>                                    InvoiceNo = invoices.InvoiceNo,
>>                                    Account = invoices.Account,
>>                                    Hidden = invoices.Hidden,
>>                                    Descrip = invoices.Descrip1,
>>                                    Created = invoices.DateTime,
>>                                    Finalized = invoices.Finalized,
>>                                    Balance = (decimal?)rt.Extension??0
>>                                });

>>
>>but this is still not what I want as I want sum of extension. Still struggling.
>
>

>      var l = from inv in invoice
>                        join ib in invoiceBalance on inv.id equals ib.id
>                        group ib by inv.id into g
>                        select new {InvoiceId = g.Key, Balance = g.Sum(s=>s.balance)};

>

I was thinking about this syntax, but my case is more complex as I want lots of columns from the parent table, not just one InvoiceNo column.

That's the reason the original query was written as a subquery (so we don't need to group on every column from the parent table).