Apex script unhandled trigger exception Salesforce

Question

I've just received a developer script exception email for a trigger I've got in a production environment:

Apex script unhandled trigger exception by user/organization: 00590000002GfMD/00D90000000cIze
SetContactDonorCampaign: execution of BeforeInsert
caused by: System.DmlException: Update failed. First exception on row 0; first error:
MISSING_ARGUMENT, Id not specified in an update call: []
Trigger.SetContactDonorCampaign: line 25, column 1

The error doesn't give me any id for the record it failed on, which is a bit annoying. The trigger itself just recognises when a Payment_Information__c record is created with the lookup Contact__c populated, and sets a multi-select picklist on that Contact to include "Donor" if it doesn't already.

trigger SetContactDonorCampaign on Payment_Information__c (before insert,before update) {

    for(Payment_Information__c donation : Trigger.new)
    {
        Contact reg = new Contact();
        if(donation.Contact__c != NULL)
        {
            reg = [SELECT id, Campaign__c FROM Contact WHERE Id = :donation.Contact__c];
            String regCampaign = '';
            if (reg.Campaign__c != NULL)
            {
                if (!reg.Campaign__c.contains('Donor'))
                {
                  regCampaign = reg.Campaign__c + ';Donor';
                } else {
                    return;
                }
            } else {            
                regCampaign = 'Donor';
            }
            reg.Campaign__c = regCampaign;
        }
        if (reg != NULL)
        {
            update reg;
        }
    }   
}

Without a specific record to look up (the system doesn't even have a Payment Information record with modified date matching this error), any suggestions on what's failing here?

Answer 1

Contact object is not null even donation.Contact__c is null. because you created new contact object each time.

Contact reg = new Contact();

if (reg != NULL)
{
        update reg;
}    

above condition is always executed. That's way this error is occur.

Please change your Code to bellow.

trigger SetContactDonorCampaign on Payment_Information__c (before insert,before update) {

for(Payment_Information__c donation : Trigger.new)
{
    Contact reg = null;
    if(donation.Contact__c != NULL)
    {
        reg = [SELECT id, Campaign__c FROM Contact WHERE Id = :donation.Contact__c];
        String regCampaign = '';
        if (reg.Campaign__c != NULL)
        {
            if (!reg.Campaign__c.contains('Donor'))
            {
              regCampaign = reg.Campaign__c + ';Donor';
            } else {
                return;
            }
        } else {            
            regCampaign = 'Donor';
        }
        reg.Campaign__c = regCampaign;
    }
    if (reg != NULL)
    {
        update reg;
    }
}   

}