Upsert and External id specification in APEX DML

Question

I am trying to upsert a list of records and have specified external id field as matching field

list<merchandise__c> m = new list<merchandise__c>();

for(integer i = 0;i<2;i++)
{
m.add(new merchandise__c(name='t'+i,price__c=12,description__c='asdf',extid__c='31'));
}

upsert m extid__c ;

Above will give me an error saying,DUPLICATE VALUE .(and yes i do have a record in DB with external id =31)

But strangely it updates that existing record without any exception when i modify the same code below as

list<merchandise__c> m = new list<merchandise__c>();

for(integer i = 0;i<2;i++)
{
m.add(new merchandise__c(name='t'+i,price__c=12,description__c='asdf',extid__c='3'+i));//changed to 3+i
}

upsert m extid__c ;

Why so?

Answer 1

You're adding two new records to your list m with the same extid__c value.

I would suggest using a map instead.

Map<String, Merchandise__c> m = new Map<String, Merchandise__c>();

for (Integer i = 0; i < 2; i++) {

    m.put(
        '31',
        new Merchandise__c (
            Name = 't' + i,
            Price__c = 12,
            Description__c = 'asdf'
        );
    );  
}

upsert m.values() extid__c;

Answer 2

In the first example you are going through the loop 2x and incrementing i at each pass, but you are explicitly assigning the extid__c with '31'. In the 2nd example you are again looping 2x and incrementing i at each pass, however you are assigning the value of '3' + i, which in the first pass would be 30 and the second pass be 31. After running the 2nd example both records with external id 30 and 31 should have been updated.