Job Reference when searching for Invoices

@operator

I am doing a transaction search for type invoice. There is a job field on the invoice type, but every one is null. How might we find the job associated with an invoice. Thanks

Here's the simple code.

var basic = new TransactionSearchBasic();
basic.type = new SearchEnumMultiSelectField
    {

        searchValue = new String[] { RecordType.invoice.ToString() },
        @operator = SearchEnumMultiSelectFieldOperator.anyOf,
         operatorSpecified = true
   };

//get all invoices (all job fields are null)
var list = _service.search(basic);