trigger leadPromotion on Lead (before insert, before update) {

   //For all the leads inserted or updated capture the Source Code
   Set<String> sourceCodes = new Set<String>();
   for(Lead l : Trigger.new){
    sourceCodes.add(l.Source_Code__c); 
   }
   
   //Query all Campaign/Promotions that have the Source Codes on the leads
   Map<Id,Campaign> campaignMap = new Map<Id,Campaign>([select Id, Source_Code__c From Campaign where Source_Code__c IN :sourceCodes]);
 
   //Create and populated a Source Code -> CampaignID Map
   Map<String,ID> sourceCodeToCampaignMap = new Map<String,ID>();
   for(Campaign c : campaignMap.values()){
    sourceCodeToCampaignMap.put(c.Source_Code__c,c.Id);
   }
   
   //For all of the leads being inserted or updated retrieve the Id of the corresponding Promotion and set it to the Primary Promotion field (lookup)
   for(Lead l : Trigger.new){
    if(l.Source_Code__c == null){ 
     l.Primary_Promotion__c = null; 
    }else{
     l.Primary_Promotion__c = sourceCodetoCampaignMap.get(l.Source_Code__c); 
    }
   }
}