The checkbox is not updating properly when trigger is called

Question

I have a trigger which will update checkbox on a child object when Update is made on Parent for mLookup__c field. But the checkbox is always false even after the trigger is executed.

Can anyone please help me with this ?

Trigger:

trigger updatefield on customObj__c (before update)
{
    Set<ID> Ids = new Set<ID>();

    for(customObj__c cb : trigger.new){

        Ids.add(cb.ID);

        }
    List<customObj__c> cbList = [Select ID,Name,mLookup__c from customObj__c where ID IN:Ids];
    List<Product__C> prodlist = [Select Id,checkbox__c,relatedCB__Lookup__c from Product__C where relatedCB__Lookup__r.Id IN: Ids];
    List<Product__C> updateList = new List<Product__C>();

    for(customObj__c op: cbList){

        if(!prodlist.isEmpty()){

        for(Product__C prod :prodlist){

        if(op.mLookup__c != null){
            prod.checkbox__c = true;

           }
           // when checked in debug it is always entering this condition
           if(op.mLookup__c == null){

            prod.checkbox__c = false;

           }
           updateList.add(prod);
        }
    }
    }
    Update updateList;
}

Answer 1

I think this could be more efficient without using extra for loop.

trigger updatefield on customObj__c (before update)
{
    List<Product__C> prodlist = [Select Id,checkbox__c,relatedCB__Lookup__c, relatedCB__Lookup__r.mLookup__c 
    FROM Product__C where relatedCB__Lookup__r.Id IN: trigger.newMap.keySet()];
    if(!prodlist.isEmpty())
    {
        for(Product__C prod :prodlist)
        {
            if(prod.relatedCB__Lookup__r.mLookup__c != null)
            {
                prod.checkbox__c = true;
            }           
            else
            {
                prod.checkbox__c = false;
            }
        }
    }
    update prodlist;
}

Answer 2

You have made thing complicated. Here is a simplified code for you

trigger updatefield on customObj__c (after update)
{

    List<Product__C> prodlist = [Select Id,checkbox__c,relatedCB__Lookup__c,elatedCB__Lookup__r.mLookup__c  from Product__C where relatedCB__Lookup__c IN: trigger.newMap().Keyset()];

    for(Product__C prod : prodlist){
        if(prod.elatedCB__Lookup__r.mLookup__c != null)
            prod.checkbox__c = true;
        else
            prod.checkbox__c = false;

    }
    Update prodlist;
}

This will work for you.

Note: You can also use process builder to achieve the same functionality or a formula field to display checked or unchecked based on lookup.