feat: add event date to response of order for admin sales revenues (formerly fees) (fossasia#6135)

Author: Anupam-dagar

app/api/admin_sales/fees.py

@@ -25,6 +25,7 @@ class Meta:
     maximum_fee = fields.Float(attribute='maximum_fee')
     revenue = fields.Method('calc_revenue')
     ticket_count = fields.Method('calc_ticket_count')
+    event_date = fields.Method('get_event_date')
 
     @staticmethod
     def calc_ticket_count(obj):
@@ -37,6 +38,10 @@ def calc_revenue(obj):
         return sum(
             [o.get_revenue() for o in obj.orders if o.status == 'completed'])
 
+    @staticmethod
+    def get_event_date(obj):
+        return obj.starts_at.isoformat()
+
 
 class AdminSalesFeesList(ResourceList):
     """