I am trying To write a trigger in which i have below requirement .i Copied this code and trying to apply on Account And Contact [closed]

Question

Trigger to Sum values (like Lookup relationship)

trigger NewLucy on Contact (after insert, after delete, after update) {
   if(Trigger.isInsert){
      try {
         for (Contact co : Trigger.new) {
            Account po = [SELECT Id,SumAllContactCurrency__c  FROM Account WHERE Id = :co.AccountId];
            List<Contact> l_co = [SELECT Id,amount__c FROM Contact WHERE AccountId = :po.Id];
            for(Contact am_co : l_co) {
               amount += am_co.Amount__c;  
            }
            po.SumAllContactCurrency__c = amount;
            update po;
         }
      } catch (Exception e) {
         System.debug(e);
      }
   }

   if(Trigger.isAfter) {
      if(Trigger.isUpdate) {
         try {
            for (Contact co : Trigger.old) {
               Account po = [SELECT Id,SumAllContactCurrency__c  FROM Account WHERE Id = :co.AccountId];
               List<Contact> l_co = [SELECT Id,amount__c FROM Contact WHERE AccountId = :po.Id];
               for(Contact am_co : l_co) {
                  amount += am_co.Amount__c;  
               }
               po.SumAllContactCurrency__c = amount;
               update po;
            }
         } catch (Exception e) {
            System.debug(e);
         }
      }
      
      if(Trigger.isDelete){
         try {
            for (Contact co : Trigger.old){
               Account po = [SELECT Id,SumAllContactCurrency__c  FROM Account WHERE Id = :co.AccountId];
               List<Contact> l_co = [SELECT Id,amount__c FROM Contact WHERE AccountId = :po.Id];
               for(Contact am_co : l_co) {
                  amount += am_co.Amount__c;  
               }
               po.SumAllContactCurrency__c = amount;
               update po;
            }
         } catch (Exception e) {
            System.debug(e);
         }
      }
   }
}

Answer 1

The SOQL and DML inside loop can be taken out and by using maps, it can be handled in a better manner. Check below code for insert scenario, it will help you understand the use of maps and you can modify the update and delete scenarios accordingly

 trigger ContactTrigger on  Contact (after insert, after delete, after update) {
   if(Trigger.isInsert){
      try {
          Set<Id> relatedAccIds = new Set<id>();
          for(Contact co : trigger.new){
              if(co.accountid <> null){
                  relatedAccIds.add(co.accountId);
              }
          }
          List<Account> relatedAccountList = [SELECT Id, SumAllContactCurrency__c  FROM Account WHERE Id in :relatedAccIds];
          List<Contact> l_co = [SELECT Id, amount__c FROM Contact WHERE AccountId in :relatedAccIds];
          Map<Id, Account> accountToAmountMap = new map<Id, Account>();
          for(Contact conObj : l_co) {
              if(accountToAmountMap.containsKey(conObj.accountId)){
                  accountToAmountMap.get(conObj.accountid).SumAllContactCurrency__c += am_co.Amount__c;
              }
              else{
                  accountToAmountMap.put(conObj.accountid, new Account(id = conObj.accountid, SumAllContactCurrency__c = conObj.Amount__c));
              }
              //amount += am_co.Amount__c;  
          }
         if(accountToAmountMap <> null)
            update accountToAmountMap.value();
         }
      } catch (Exception e) {
         System.debug(e);
      }
   }