trigger TaskAccountNumber on Task (before insert, before update) {

   /***************************************************************************
   * TaskBefore Trigger - TaskCoverage 29/9/08
   * 
   * Description:
   *   Before a task is inserted/updated, look for a matching contact/lead and update 
   *   Account_Number__c on the task.
   *
   * Notes:
   *   To avoid reaching Apex code built-in limits, the code is structured to keep 
   *   the number of SOQL queries to an absolute minimum. 
   *
   * Revision History
   * none
   ****************************************************************************/  
   
// List to hold the WhoId of all incoming task records
  List<String>  whoIdList = new String [] {};
   for(Integer i = 0; i<Trigger.new.size(); i++) {
        if(Trigger.new[i].WhoId != null){
            if(Trigger.isInsert){
                whoIdList.add(Trigger.new[i].WhoId);
            }else{
                if(Trigger.new[i].WhoId != Trigger.old[i].WhoId){
                    whoIdList.add(Trigger.new[i].WhoId);
                }
            }
        }else{
            Trigger.new[i].Account_Number__c = null;
        }
    }
    //issue one query to get all contacts for all incoming who ids (to avoid Apex restrictions)
    List<Contact> matchingContacts = [SELECT Id, Account_Number__c from Contact where Id IN :whoIdList];
    // issue one query to get all leadsfor all incoming who ids (to avoid Apex restrictions)
    List<Lead> matchingLeads =  [SELECT Id, Demo_Account__c from Lead where Id IN :whoIdList];
    
    Map<String, Contact> foundContacts = new Map<String, Contact>();  
    for (Contact loopContact : matchingContacts) {
        foundContacts.put(loopContact.Id, loopContact);
    }
  
    Map<String, Lead> foundLeads = new Map<String, Lead>();  
    for (Lead loopLead : matchingLeads) {
        foundLeads.put(loopLead.Id, loopLead);
    }
    // loop through the incoming tasks again, for each, loop through the found contacts/leads to find matching record
    // on Id
    for(Task loopTask : Trigger.New) {
        if (loopTask.WhoId != null) {
          Contact foundContact = foundContacts.get(loopTask.WhoId);
          if (foundContact != null) {
             loopTask.Account_Number__c = foundContact.Account_Number__c;
          }else{
              Lead foundLead = foundLeads.get(loopTask.WhoId);
                if(foundLead != null){
                    loopTask.Account_Number__c = foundLead.Demo_Account__c;
                }
          }
        }
    }    
}