Hi, 

I need help to do a DML operation with external id field. I am using the below syntax, but it is throwing an error

Schema.SObjectField unique_projectuser = CC_Project_User__c.Fields.Unique_Project_User__c;
 List<Database.UpsertResult> res = Database.Upsert(projectUsers_List, unique_projectuser ,false);  

Error: getErrors=(Database.Error[getFields=();getMessage=Unique_Project_User__c not specified;getStatusCode=MISSING_ARGUMENT;]);getId=null

ANy help here is greatly appreciated. Thanks