Using Trigger.new List and Map compare new contracts with old contracts

Question

I'm trying to compare dates of old contracts and new contracts (trigger.new) from the same account, the new contracts are imported and are from a lot of different accounts. But the performance of it has to be better. So I was thinking to do a map of old contracts that compare by the AccountId and not iterate all contracts every time. Below is my code so far. Is it possible to do it using a map, maybe like Map <Id, List<Contract>> ? And how I iterate over a map like this one?

public class NewContracts {

public static void comparingContracts(List<Contract> newContracts){

    Set<String> setAccounts = new set<String>();
    list<Contract> oldContracts = new list<Contract>();


    for (Contract c: newContracts){
        setAccounts.add(c.AccountId);
    } 

    oldContracts = [SELECT AccountId, EndDate, RecordTypeId FROM Contract WHERE AccountId in :setAccounts];

    for(Contract oCon: oldContracts){

        for(Contract nCon: newContracts){                            
            if (oCon.AccountId == nCon.AccountId ){
                nCon.checkTest__c= compareDate(oCon.EndDate, nCon.StartDate);   
            }               
        }  
    }  
}

}

Answer 1

Yes, you can do that. It requires a three step process that I call the Aggregate Query Update pattern.

public static void comparingContracts(List<Contract> newContracts) {
    Map<Id, Contract[]> oldContracts = new Map<Id, Contract[]>();
    // Aggregate
    for(Contract record: newContracts) {
        oldContracts.put(record.AccountId, new Contract[0]);
    }
    // Query 
    for(Contract record: [SELECT AccountId, EndDate, RecordTypeId FROM Contract WHERE AccountId in :oldContracts.keySet()]) {
        oldContracts.get(record.AccountId).add(record);
    }
    // Update
    for(Contract record: newContracts) {
        for(Contract oldRecord: oldContracts.get(record.AccountId)) {
            record.CheckTest__c = compareDate(oldRecord.EndDate, record.StartDate);
        }
    }
}