Fix bugs and improvements

fix payment outstanding with amount <= 0
fix payment outstanding with other payments

app/models/shift_sale.rb

@@ -19,7 +19,7 @@ class ShiftSale < ApplicationRecord
 
   def self.current_shift
     # today_date = DateTime.now.strftime("%Y-%m-%d")
-    shift = ShiftSale.where("shift_started_at is not null and shift_closed_at is null").first
+    shift = ShiftSale.where.not(shift_started_at: nil).where(shift_closed_at: nil).first
     return shift
   end