DUPLICATE_VALUE error - Composite REST API by External Id

Question

I'm trying to upsert a record using an external Id with composite API.

my external Id is (External ID) (Unique Case Insensitive) - to be precise - I created this field only with the external Id checkbox checked, and after some time I turned it also to be unique.

When I'm trying to update a value that already exists in my system using upsert - (even if it only one record like the JSON below) I'm getting an error :

Duplicate value found: customObject__c duplicates value on record with id: xxxxxxxx

Which of course I have only one record with that current external Id.

After some investigation, I found out that if I'm creating a new record Its Inserted or updated as expected when using the same call.

But when I'm dealing with a record that was in my DB before I turned the external Id field to be unique, I'm getting this above error.

What I'm doing :

https://xxx.salesforce.com/services/data/v50.0/composite/sobjects/customObject__c/CustomExternalIdField__c

{
    "allOrNone": false,
    "records": [{
        "CustomExternalIdField__c":"503",
        "field1__c":"RS",
        "field2__c":"PN",
        "field3__c":"PEN",
        "Name":"Name",
        "field4__c":"RE",
        "attributes": {
            "type": "customObject__c"
        }

    }]
}

P.S

I'm doing this API call on 5 different Objects which all of them have an externalId that changed to be also unique over time - and Its working as excepted.

Any Ideas please?

Answer 1

In case it helps anyone who has the same problem: I had a similiar situation in my org today, and I found out that the object has a before insert trigger which changes the value of the external id field.

I was doing a Patch rest api with "external_id__c" = "example1", and in the before insert trigger, apex changed the value external_id__c = "example2". The first time the REST PATCH worked, inserting the record. But in the following calls what happens is: REST searches for "external_id__c" = "example1" for update >> it does not find any match >> it tries to insert the record >> before insert trigger changes the value external_id__c = "example2" >> rest returns ERROR Duplicate value found: "customObject__c duplicates value on record with id: xxxxxxxx".