count and update no. of record in another object

Question

There is a lookup relationship between two object CountParent__c and CountChild__c.

CountParent__c object contains a custom field called NoOfChildRecords__c which will count the no. of CountChild__c records associated with it.

Required scenario is,
If user edited a CountChild__c record and changes the value of lookup field from P2-->P1 then NoOfChildRecords__c should be increased by 1 in P1 and decreased by 1 in P2.

all this happening on before update trigger.

if (Trigger.isUpdate && Trigger.isBefore) {
    List < CountParent__c > incCount;
    Set < id > triggerIds = Trigger.oldMap.keySet();
    List < CountChild__c > fetchChild = [select Name, CountParent__r.name from CountChild__c where Id IN: triggerIds];
    system.debug('old====' + fetchChild);
    Set < id > triggerIds1 = Trigger.newMap.keySet();
    List < CountChild__c > fetchChild1 = [select Name, CountParent__r.name from CountChild__c where Id IN: triggerIds1];
    system.debug('new====' + fetchChild1);
    List < CountParent__c > fetchParent = [select Name, NoOfChildRecords__c from CountParent__c];
    for (CountChild__c a: fetchChild1) {
        for (CountParent__c b: fetchParent) {
            if (b.Name == a.CountParent__r.Name) {
                incCount = new List < CountParent__c > ();
                b.NoOfChildRecords__c = b.NoOfChildRecords__c + 1;
                incCount.add(b);
            }
        }
    }
    update incCount;
}

this is the code i was trying but seems wrong.

thanks!

Answer 1

This code may help to you

if (Trigger.isUpdate && Trigger.isAfter) {

    Set<id> parentIds = new Set<Id>();
    List<CountParent__c> parentList = new List<CountParent__c>();
    Map<Id,Integer> parentIdAndCountMap = new Map<Id,Integer>();

    for(CountChild__c child : trigger.new) {

        if(child.CountParent__c != null && Trigger.oldMap.get(child.Id).CountParent__c != child.CountParent__c) {

            parentIds.add(child.CountParent__c);
        }
    }

    if(parentIds.size() > 0) {
        for(CountChild__c c : [SELECT Id, Name, CountParent__c FROM CountChild__c WHERE CountParent__c IN :parentIds]) {
            if(!parentIdAndCountMap.containsKey(c.CountParent__c))
                parentIdAndCountMap.put(c.CountParent__c, 0);
            parentIdAndCountMap.put(c.CountParent__c, parentIdAndCountMap.get(c.CountParent__c)+1);
        }
    }

    if(parentIdAndCountMap.size() > 0) {
        for(Id pId : parentIdAndCountMap.KeySet()) {
            CountParent__c prnt = new CountParent__c();
            prnt.Id = pId;
            prnt.NoOfChildRecords__c = parentIdAndCountMap.get(pId);
            parentList.add(prnt);
        }
    } 

    if(parentList.size() > 0) {
        update parentList;
    }    
}