如何在Codeignitor中向数据表查询添加自联接

Question

$ this-> datatables-> select（“ purchases.id，purchases.date，purchases.reference_no，purchase.supplier，purchases.status，purchases.grand_total，purchases.paid，（purchases.grand_total-purchases.paid）作为余额，purchases.payment_status”）                 -> from（'purchases'）-> join（'purchases as a'，'purchases.id = a.return_purchase_ref'）;