Example of mapping related records to report errors on (Apex Code)

TriggerDmlHelper.apex

// Help report errors to parent/child object on failed DML update
// For use in Trigger contexts only.
// This version is for demonstration purposes only, and should not
// be considered production ready without additional modifications.
// 
// Example trigger:
// 
// trigger updateContactPhone on Account(after update) {
//   Id[] updates = new Id[0];
//   for(Account record: Trigger.new) {
//	   if(record.Phone != Trigger.oldMap.get(record.Id).Phone) {
//		 updates.add(record.Id);
//	   }
//   }
//   if(!updates.isEmpty()) {
//     TriggerDmlHelper helper = new TriggerDmlHelper(List<Account>.class, List<Contact>.class);
//     for(Contact record: [SELECT AccountId FROM Contact WHERE AccountId = :updates]) {
//       Account triggerAccount = Trigger.newMap.get(record.AccountId);
//       record.Phone = triggerAccount.Phone;
//       helper.addMapping(triggerAccount, record);
//     }
//     helper.updateRecords('Failed to update {1}. First error was: {2}');
//   }
// } 

public class TriggerDmlHelper {
    // Class to normalize various database.*result types.
    class DmlResult {
    	Id recordId;
    	Integer rowIndex;
    	Boolean success;
    	Database.Error[] errors;
    	DmlResult(Boolean success, Id recordId, Database.Error[] errors, Integer rowIndex) {
    		this.success = success;
    		this.recordId = recordId;
    		this.errors = errors;
    		this.rowIndex = rowIndex;
    	}
    }
    // normalizes the various database.*result types for reportErrors
    class DmlResultList implements Iterable<DmlResult> {
    	DmlResult[] genResults;
    	DmlResultList(Database.SaveResult[] results) {
    		genResults = new DmlResult[0];
			for(Database.SaveResult result: results) {
				genResults.add(new DmlResult(result.isSuccess(), result.getId(), result.getErrors(), genResults.size()));
			}
    	}
    	DmlResultList(Database.DeleteResult[] results) {
    		genResults = new DmlResult[0];
			for(Database.DeleteResult result: results) {
				genResults.add(new DmlResult(result.isSuccess(), result.getId(), result.getErrors(), genResults.size()));
			}
    	}
    	DmlResultList(Database.UndeleteResult[] results) {
    		genResults = new DmlResult[0];
			for(Database.UndeleteResult result: results) {
				genResults.add(new DmlResult(result.isSuccess(), result.getId(), result.getErrors(), genResults.size()));
			}
    	}
    	public Iterator<DmlResult> iterator() {
    		return genResults.iterator();
    	}
    }

    // 1:1 mapping of which record is trying to update another record
    SObject[] source, dest;

    // Recommend using concrete types like List<Account> instead of List<SObject>
    public TriggerDmlHelper(Type sourceType, Type destType) {
        source = (SObject[])sourceType.newInstance();
        dest = (SObject[])destType.newInstance();
    }

    // Adds another 1:1 mapping
    public void addMapping(SObject src, SObject dst) {
        source.add(src);
        dest.add(dst);
    }
    
    // DML operations
    public void insertRecords(String msg) {
        reportErrors(new DmlResultList(Database.insert(dest, false)), msg);
    }
    public void updateRecords(String msg) {
        reportErrors(new DmlResultList(Database.update(dest, false)), msg);
    }
    public void deleteRecords(String msg) {
        reportErrors(new DmlResultList(Database.delete(dest, false)), msg);
    }
    public void undeleteRecords(String msg) {
        reportErrors(new DmlResultList(Database.undelete(dest, false)), msg);
    }

    // For each failure, add an error to the source record
    void reportErrors(DmlResultList results, String msg) {
        Iterator<DmlResult> iter = results.iterator();
        while(iter.hasNext()) {
            DmlResult result = iter.next();
            if(!result.success) {
                source[result.rowIndex].addError(
                    String.format(
                        msg,
                        new String[] {
                            source[result.rowIndex].Id,
                            dest[result.rowIndex].Id,
                            result.errors[0].getMessage()
                        }
                    )
                );
            }
        }
    }
}

Great example. I bookmarked it for future use.

I think the below line is missing 0 argument:

helper.updateRecords('Failed to update {1}. First error was: {2}');