Update using External Id

Question

I understand UPDATE operation doesn't support ExternlID.

Is there any other way i can use Dataloader to update Salesforce records using external id ?

If i use UPSERT operation, i don't want to insert records where a matching ExternlId is not found. I am thinking I could use a validation rule to present inserting records when no ExternalId is found. However, i don't what should the criteria be to prevent inserting records?

Does anyone have suggestions?

Answer 1

I usually use the trick of creating a simple validation rule

AND(
  ISNEW(),
  $User.Alias = "alias-of-user-doing-upsert"
)

Then put ALL_OR_NONE to false, and from all the errors, filter the onew with the message you write on the validation rule. (so the rest of them would actually be update failures)