Upsert DML Operation

Question

Consider the below code. I thought the way upsert DML works is that if it sees that there is a new record with matching key (here in this case there is already an account with Name = Acme and BillingCity = Austin), it will not insert the new account in line number 6 and will just update the existing account with the Phone = '412 9012333'. But however it doesn't work that way and it inserts the account if it finds a new Account statement. Please correct me if my understanding is correct and whether I am missing something.

1 List<Account> newAcct = new List<Account>();
2 newAcct = [SELECT Id, Name, BillingCity  FROM Account WHERE Name = 'Acme'];
3 for(Account c : newAcct){
4    c.BillingCity = 'Austin';
5 }
6 Account newAcct2 = new Account(Name ='Acme', BillingCity = 'Austin', Phone = '412 9012333');
7 newAcct.add(newAcct2);
8 upsert newAcct;

Answer 1

That's not how upsert works. It works by using Id, or an External ID, to determine if the record should be inserted or updated. You would need some sort of external ID field to work with:

List<Account> newAcct = [SELECT Id, Name, BillingCity  FROM Account WHERE Name = 'Acme'];
Integer externalIdCounter = 0;
for(Account c : newAcct) {
  c.External_Id__c = (externalIdCounter++)+'';
}
update newAcct;
Account newAcct2 = new Account(External_Id__c='0', Name ='Acme', BillingCity = 'Austin', Phone = '412 9012333');
newAcct.add(newAcct2);
upsert newAcct Account.External_Id__c;

Upsert does not use multiple keys/composite keys. It requires you to set up a special field for the purpose of upserting, or using the Salesforce ID as the key (which is the default if you do not specify a field).