Hi Anton,

Try the below trigger on objectA__c
 

trigger UpdateParentCount on ObjectA__c  (after insert, after update, after delete, after undelete) {

   List<ObjectB__c> ObjectBList = new List<ObjectB__c>();   
   Set<Id> ObjectAids = new Set<Id>();
   
   if(Trigger.isDelete) {
     for(ObjectA__c  test:Trigger.Old) {      
        ObjectAids.add(test.ObjectB__c);   
         }      
   }

   else if(Trigger.isUpdate) {
     for(ObjectA__c  test:Trigger.New) {      
        ObjectAids.add(test.ObjectB__c);     
     }

     for(ObjectA__c  test:Trigger.Old) {      
        ObjectAids.add(test.ObjectB__c);       
     }      
   }
   else{
     for(ObjectA__c  test:Trigger.New) {      
        ObjectAids.add(test.ObjectB__c);    
     }
   }
   
   AggregateResult[] groupedResults = [SELECT COUNT(Id), ObjectB__c FROM ObjectA__c  
   where ObjectB__c IN :ObjectAids GROUP BY ObjectB__c ];
   
   for(AggregateResult a:groupedResults) {     
     Id BId = (ID)a.get('ObjectB__c');     
     Integer count = (INTEGER)a.get('expr0');     
     ObjectB__c objB = new ObjectB__c(Id=BId);     
     objB.CountUsed__c  = count;     
     ObjectBList.add(objB);      
   }   
   
   update ObjectBList;

}

Let me know if you have any issues.

Regards

Hi Anton,

You can achieve it in two ways.

1 - Convert lookup relationship to Master Detail and then create a Rollup Summary field in ObjectB__c which will count ObjectA__c Object records.
2 - Create a Number field in ObjectB__c which will store the total count of ObjectA__c and then create a new formula field which will hold the newly create field value then add this formula field into the layout and remove Number filed. This way you will find a field which is read-only.
3 -  Built a trigger which will be responsible for counting ObjectA__c records and then updating ObejectB__c records with the total value.

Sample code for the trigger is given below and in the below code I am counting no of Attachments into Account Object.
 

trigger trgCountAttachments on Attachment (After insert, After delete, After undelete) {
    Set<Id> parentIdsSet = new Set<Id>();
    List<Account> accountListToUpdate = new List<Account>();
    IF(Trigger.IsAfter){
        IF(Trigger.IsInsert || Trigger.IsUndelete){
            FOR(Attachment a : Trigger.new){
                if(a.ParentId!=null){   
                   parentIdsSet.add(a.ParentId); 
                }
            }
        }
        IF(Trigger.IsDelete){
            FOR(Attachment a : Trigger.Old){
                if(a.ParentId!=null){   
                   parentIdsSet.add(a.ParentId); 
                }
            }
        }
    }
    System.debug('#### parentIdsSet = '+parentIdsSet);
    List<Account> accountList = new List<Account>([Select id ,Name, No_Of_Attachments__c, (Select id, Name From Attachments) from Account Where id in:parentIdsSet]);
    FOR(Account acc : accountList){
        List<Attachment> attachmentList = acc.Attachments;
        acc.No_Of_Attachments__c = attachmentList.size();
        accountListToUpdate.add(acc);
    }
    try{
        update accountListToUpdate;
    }catch(System.Exception e){
        
    }
}

 

Hi Anton,

Try with below code.

trigger UpdateParentCount on ObjectA__c  (after insert, after update, after delete, after undelete) {

   List<ObjectB__c> ObjectBList = new List<ObjectB__c>();   
   Set<Id> ObjectAids = new Set<Id>();
   
   if(Trigger.isDelete) {
     for(ObjectA__c  test:Trigger.Old) {      
        ObjectAids.add(test.ObjectB__c);   
         }      
   }else if(Trigger.isUpdate || Trigger.IsInsert() || Trigger.IsUndelete()) {
     for(ObjectA__c  test:Trigger.New) {      
        ObjectAids.add(test.ObjectB__c);     
     }     
   }
   AggregateResult[] groupedResults = [SELECT COUNT(Id), ObjectB__c objBId FROM ObjectA__c  
   where ObjectB__c IN :ObjectAids GROUP BY ObjectB__c ];
   
   for(AggregateResult a:groupedResults) {     
     Id BId = (ID)a.get('objBId');     
     Integer count = (INTEGER)a.get('expr0');     
     ObjectB__c objB = new ObjectB__c();
     objB.id = BId;
     objB.CountUsed__c  = count;     
     ObjectBList.add(objB);      
   }   
   
   update ObjectBList;

}

Let me know if this do the trick :)

Thanks,
Amit Singh.

Cheers Amit!
Just at line 10 isInsert and isUndelete are no functions, I removed the "()" part.

Working as intended except for 1 situation:
deleting the field value without deleting the record.

inserting a new ObjectA record, updating it with a ObjectB value, deleting the whole ObjectA record are all good.
However when I try to delete the ObjectB value without deleting the ObjectA record I get the following error:

MISSING_ARGUMENT, Id not specified in an update call: [] Trigger.UpdateParentCount: line 27, column 1