Krdytkyu

The trigger is firing, there are no errors, and it is running through all of the code, except the FOR loop w/ the SOQL query.

THE QUESTION
Why is the trigger not jumping into the FOR loop?

THE TRIGGER

trigger LookupRollup on Child__c (after insert, after update, after delete, after undelete) {



    // List of parent record ids to update

    Set<Id> parentIds = new Set<Id>();



    // In-memory copy of parent records

    Map<Id, Opportunity> parentRecords = new Map<Id, Opportunity>();



    // Gather the list of ID values to query on

    for (Child__c c : Trigger.isDelete?Trigger.old:Trigger.new) {

        parentIds.add(c.Opportunity__c); 

    }



    // Avoid null ID values

    parentIds.remove(null);



    // Create in-memory copy of parents

    for (Id parentId:parentIds) {

        parentRecords.put(parentId, new Opportunity (Id = parentId, Child_Rollup__c = 0));    

    }



    // Query all children for all parents, update Rollup Field value

    for (Child__c c : [select id, Dollar__c, Opportunity__c

                       from Child__c

                       where id in :parentIds]) {



                parentRecords.get(c.Opportunity__c).Child_Rollup__c += c.Dollar__c;

    }



    // Commit changes to the database

    Database.update(parentRecords.values());



}

Because you're querying against Id with a Set of Ids of the wrong object. The query result is empty, so the for loop does not execute - it has nothing to iterate over.

parentIds contains Opportunity Ids:

    parentIds.add(c.Opportunity__c); 

Your query is

select id, Dollar__c, Opportunity__c

                   from Child__c

                   where id in :parentIds

No Child__c record's Id is in parentIds.

Instead, it appears that you want to query for Child__c records

WHERE Opportunity__c IN :parentIds

As cropredy points out in a comment, you can save yourself from reinventing the wheel here by simply using Declarative Lookup Rollup Summaries.

Alternatively you can use an aggregate function, This will be usefull if you have more than few hundred childs per parent, and you can run into SOQL rows limits if that happens.

trigger LookupRollup on Child__c (after insert, after update, after delete, after undelete) {



    // List of parent record ids to update

    Set<Id> parentIds = new Set<Id>();



    // In-memory copy of parent records

    Map<Id, Opportunity> parentRecords = new Map<Id, Opportunity>();



    // Gather the list of ID values to query on

    for (Child__c c : Trigger.isDelete?Trigger.old:Trigger.new) {

        parentIds.add(c.Opportunity__c); 

    }







    AggregateResult groupedResults =[SELECT Opportunity__c,sum(Dollar__c) FROM Child__c 

                                        WHERE Opportunity__c in :parentIds GROUP BY Opportunity__c];



    for(AggregateResult ar: groupedResults){

        parentRecords.get(ar.get('Opportunity__c')).Child_Rollup__c = ar.get('expr0); ;

    }



    // Commit changes to the database

    Database.update(parentRecords.values());



}

Src : https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/langCon_apex_SOQL_agg_fns.htm