Apex Trigger for Lead Assignment

Question

I'm trying to create a trigger that will automatically re-route a lead through the default routing rules when a custom field checkbox is checked. I have the following code based on a suggestion by product mgmt through the ideas forum:

trigger reassignLeads on Lead (before update){
  List<Lead> leads=new List<Lead>();
  Database.DMLOptions dmo=new Database.DMLOptions();
  dmo.assignmentRuleHeader.UseDefaultRule=True;
  for(Lead l:trigger.new){
    if(l.Re_Run_Assignment_Rules__c=True){
      l.Re_Run_Assignment_Rules__c=False; //set it to false to avoid continually reassigning this lead
      l.setOptions(dmo);
      leads.add(l);
    }
  }
  Database.Update(leads);
}

However, when I try to test this out on a lead, I receive the following error:

Apex trigger reassignLeads caused an unexpected exception, contact your administrator: reassignLeads: execution of BeforeUpdate caused by: System.SObjectException: DML statment cannot operate on trigger.new or trigger.old: Trigger.reassignLeads: line 12, column 1

I've also used the following code:

trigger reassignLeads on Lead (before update)
{
    For (lead l:trigger.new){
        Database.DMLOptions dmo = new Database.DMLOptions();
        dmo.assignmentRuleHeader.useDefaultRule= true;
        if (l.Re_Run_Assignment_Rules__c=True)
            l.setOptions(dmo);
        Database.update(l);
    }
}

This code doesn't cause an error when I try to update a lead but it also doesn't re-route the lead. I assume, because it's using before update instead of after update. If I change trigger to after update, I get the following error:

Apex trigger reassignLeads caused an unexpected exception, contact your administrator: reassignLeads: execution of AfterUpdate caused by: System.FinalException: Record is read-only: Trigger.reassignLeads: line 6, column 1

Is there a way to do this using a trigger or do I have to use a future call to a class?

Answer 1

And to provide some color commentary for others based on Naren's answer, here's an example

@future
public static void futureLeadReassign(Set<ID> lIdReassignSet) {

  List<Lead> lUpdList        = [SELECT Id FROM Lead WHERE Id IN: lIdReassignSet];
  for (Lead l:lUpdList){
    Database.DMLOptions dmo         = new Database.DMLOptions();
    dmo.assignmentRuleHeader.useDefaultRule  = true;    // use leadAssignment rules when updating
    l.setOptions(dmo);
  }

  try {update(lUpdList);}
  catch (DmlException e) { // do something useful like emailing the sysad or logging to sobject }
}

Answer 2

The reassign method should be asynchronous. Create an apex class with @future method and pass your ids from the trigger to this class.