How do I validate new records with old records?

Question

I have a custom object O__c it has Lookup relationship to Opportunity. This Custom Object record will create when oppty stage is moved from 95% to 100% according to our requirement.

Now my question is I have 300 Opportunities already existing which has O__c Object record Each.

There is a custom field in my custom object and every time when I add new record it should validate me such a way that the recent record's custom field should be equal to it's earlier records. How can I do that?

Answer 1

You can do SOQL in a trigger, so just query each of your parent opportunity's child records and compare:

Set<ID> parentOpps = new Set<ID>();
for(O__c o : Trigger.New)
{
    parentOpps.add(o.Opportunity__c);
}

Map<ID, Opportunity> opps = new Map<ID, Opportunity>(
     [select ID, (select ID, Custom_Field__C from Os__r) from Opportunity
      where ID in :parentOpps]
);

for(O__c newO : Trigger.New)
{
    for(O__c existingO : opps.get(newO.Opportunity__c).Os__r)
    {
        if(existingO.Custom_Field__c != newO.Custom_Field__c)
        {
            newO.addError('Custom Field does not match');
            break;
        }
    }
}

Also, something this doesn't consider is the case of two or more O__c records being inserted at once to an Opportunity with no child records.