Aggregate-Query-Update pattern (AQU) for updating child records dates with date record on parent

Question

I have a custom object called Location__c that has a date field called Active_Date__c. Location__c has a child object called Cans__c. Cans__c has a date field Start_Date__c. I would like to write a trigger that populates the child dates with the master date. I am very new to this, so be gentle. Here is what I have so far:

trigger addActiveDateToChildStartDate on Location__c(after insert, after update){

    Can__c[] canList = new can__c[0];
    Map <Id,Can__c> servicesMap = new Map<id,Can__c>();

    locations.putAll([SELECT Id,Active_Date__c,(SELECT Id, Start_Date__c FROM Cans__r)FROM Location__c WHERE Id In :locations.keySet()]);
    for (Location__c Loc: locations){
        List<Can__c> services = Loc.getSObjects('Can__c');
        servicesMap.put(Loc.Id, Can__c);
    }   
}

Any help would be appriciated, Thanks!

Answer 1

Try something like this. This is making a few assumptions on your field names but this should give you the general idea.

All you need to do is query the child records that are related to the Location objects in the trigger context. From there, you can just grab the Active Date of the parent Location object by using the trigger.newMap context variable.

trigger addActiveDateToChildStartDate on Location__c(after insert, after update){

    list<Can__c> childCans = [Select Id, Start_Date__c, Locations__c From Can__c Where Locations__c IN : Trigger.new];

    for(Can__c can : childCans){
         can.Start_Date__c = trigger.newMap.get(can.Locations__c).Active_Date__c;
    }
    update childCans;
}