feat(charge):基本完成所有用户服务延期记录部分的功能，待测。

repository/charge.go

@@ -4,6 +4,7 @@ import (
 	"electricity_bill_calc/config"
 	"electricity_bill_calc/global"
 	"electricity_bill_calc/model"
+	"electricity_bill_calc/utils"
 	"time"
 
 	"xorm.io/builder"
@@ -50,3 +51,20 @@ func (_ChargeRepository) ListPagedChargeRecord(keyword, beginDate, endDate strin
 		Find(&charges)
 	return charges, total, err
 }
+
+func (_ChargeRepository) LastValidChargeTo(uid string) (time.Time, error) {
+	veryBlankTime, _ := time.Parse("2006-01-02 15:04:05", "0001-01-01 00:00:00")
+	var records = make([]time.Time, 0)
+	err := global.DBConn.Where(builder.Eq{"settled": true, "cancelled": false, "refunded": false}).Cols("charged_to").Find(&records)
+	if err != nil {
+		return veryBlankTime, nil
+	}
+	lastValid := utils.Reduce(records, veryBlankTime, func(acc, elem time.Time) time.Time {
+		if elem.After(acc) {
+			return elem
+		} else {
+			return acc
+		}
+	})
+	return lastValid, nil
+}