Hello Ram,


Below is the actual code of my trigger 

/**
 * [SFDC_Class__c This trigger used to create a event in calendar when a new session record get created and the values of start and end dat will get copied to the old start and old end date]
 * @param  insert [Work on record creation]
 * @param  Update [Work on record update]
 * @return        [Null]
 * @author [Sudhir Dudeja]
 */
trigger CreateEvents on SFDC_Class__c (after insert,after Update,before update,before insert) {

       List<Event> eventslist = new List<Event>(); // to hold th events
       List<SFDC_Class__c> sessDay = new List<SFDC_Class__c>();
       List<Event> eventList = new List<Event>();
       List<SFDC_Class__c> sessionList=new List<SFDC_Class__c>(); //to hold session object
       List<Event> eventtoUpdate = new List<Event>(); //list to hold the event records to update
       public List<SFDC_Class__c> sfdcs=new List<SFDC_Class__c>(); //to hold session object
       String fullRecordURL;
       Public Datetime dd;
       Public Datetime dd1;
       List<SFDC_Class__c> sessionupdateList=new List<SFDC_Class__c>();


        if(trigger.isInsert && trigger.isBefore){
          for(SFDC_Class__c sDay : trigger.new){
            if(sDay.Create_Event__c==true && sDay.Start_Date_New__c==null) {
              sDay.Start_Date_New__c.addError('Start Date is manadtory to create event');
            }
          }
        }

        if(trigger.isInsert && trigger.isAfter){

           for(SFDC_Class__c session : Trigger.new)
           {

                  SFDC_Class__c sfdc=new SFDC_Class__c(id=Session.id);   //Creating a new session object
                  if(session.Start_Date_New__c!=null){   //if date value in start date new is not null
                    sfdc.Start_Date__c=session.Start_Date_New__c.date(); //then copy the data in the old start date
                    dd=session.Start_Date_New__c;  //create a dummy vairable which hold the new start date value
                  }
                  if(Session.End_Date_New__c!=null) //Check new end date is not null
                  {
                    sfdc.End_Date__c=session.End_Date_New__c.date(); //copy the value of new end date to old end date
                    dd1=session.End_Date_New__c;//create a dummy vairable which hold the new end date value
                  }
                    system.debug(sfdc);
                    sessionList.add(sfdc);  //Add session object to the list

                    if(session.Create_Event__c==true && session.Start_Date_New__c==null) {
                      session.Start_Date_New__c.addError('Start Date is manadtory to create event');
                    }


                  if(session.Create_Event__c==true && session.Start_Date_New__c!=null && session.End_Date_New__c!=null){ //if checkbox is checked , create event
                  Event ee=new Event(); //create a new event
                  ee.OwnerId=label.Contain_ID_of_the_data_CRM; //copy the logged in user id to event
                  ee.ActivityDateTime=session.Start_Date_New__c;
                  ee.Description='Session Event';
                  ee.EndDateTime=session.End_Date_New__c;
                  ee.IsReminderSet=True;
                  ee.IsAllDayEvent=False; //Evnt is not all day
                  ee.Instructor__c=session.Dummy_Instructor_Field__c;
                  system.debug(session.Dummy_Instructor_Field__c);
                  ee.Session_Name__c=session.Name;
                  ee.Session_Status__c='Scheduled';
                  ee.Subject=session.Name;
                  ee.Location=session.Facility__c;
                  ee.Session_ID__c=session.id;  //A variable in event to hold the session ID in the event.
                  ee.SessionDayURL__c = URL.getSalesforceBaseUrl().toExternalForm() + '/' + session.Id;  //URL of session record
                  system.debug(session.Courses_Names__c);
                  //This event field all the details in each seprate lines.
                  ee.Description='Instructor Name :'+ ' ' +session.Dummy_Instructor_Field__c+ '' + '\t\n' +''+ 'Session Name :'+ ' ' +session.Name+ '' + '\t\n' +''+ 'Course Name :'+ ' ' +session.Courses_Names__c+ '' + '\t\n' +''+ 'Location :' + ' ' +session.Facility__c+ ''+ '\t\n' +''+ 'Is Catering Required ? :'+ ' ' +session.Is_Catering_Required__c+'' + '\t\n' +'\t\n' +''+ 'Session URL :' + ' ' +URL.getSalesforceBaseUrl().toExternalForm() + '/' + session.Id+'' + '\t\n' +''+ 'Additional Requests :'+ '\t\n' +session.Additional_Request__c;
                  eventslist.add(ee); //Add events to the list @declare variable at line 10
                  system.debug(ee.Description);
                }
           }

           if(!eventslist.isempty()) //If evenlist is not empty
           {
             insert eventslist; //insert all events
           }
             if(!sessionList.isempty() && dd!=null && dd1!=null) //if session list is not empty and contains start date and end date then update session object
           {
             update sessionList; //update the session object
           }
        }

        if(trigger.isUpdate && trigger.isBefore){
          for(SFDC_Class__c sDay : trigger.new){
            if(sDay.Create_Event__c==true && sDay.Start_Date_New__c==null) {
              sDay.Start_Date_New__c.addError('Start Date is manadtory to create event');
            }
          }
        }

      if(trigger.isUpdate && trigger.isAfter){  //trigger work upon the update event
       if(preventTriggerRecursion.updatedOnce1()) //To check whether the trigger is not get stuck in recursion if we are updating the same object
        {
          Map<Id,Id> sessionIdMap = new Map<Id,Id>(); //Create a map to hold session ID and event ID
          Set<Id> SessionIdSet = new Set<Id>(); //Set to hold ID

          for(SFDC_Class__c sess : Trigger.new)
          {
            SessionIdSet.add(sess.id);  //Set to hold all the record ID whome we have to update
          }

          system.debug('SessionIdSet->' +SessionIdSet);

          //Query to event to fetch those events which are associate with session.
          eventList = [Select Id,ActivityDateTime,EndDateTime,Session_ID__c FROM Event where Session_ID__c = : SessionIdSet];

          system.debug('eventList-->'+ eventList);
          System.debug(eventList.size());

          //if event list is not null and size is greater than zero
          if(eventList != Null || eventList.size()>0){
          for(Event eve : eventList)
          {
          if(eve.Session_ID__c != Null){  //if session ID in event not null
          sessionIdMap.put(eve.Session_ID__c,eve.Id); //then add the session ID and event id to the map
          }
          }
          system.debug('sessionIdMap-->'+sessionIdMap);
          }

          //Loop to all records which we are updating
          for(SFDC_Class__c sDay : trigger.new){


            //if checkbox is checked mark true in event.
            //Trigger.oldMap.get(sDay.id).Client__c==sDay.Client__c this conidition is added to remove the error which getting on mass update button on session other than this was not necessary to put
            if(sDay.Create_Event__c==true){
                Event eve = new Event(); //Create a new event
                eve.ID = sessionIdMap.get(sDay.ID); //fetching event record or event from map
                system.debug('*****'+eve.ID);
                eve.ActivityDateTime = sDay.Start_Date_New__c;  //change the time in event
                eve.EndDateTime = sDay.End_Date_New__c;//change the end date in event
                eve.Instructor__c=sDay.Dummy_Instructor_Field__c; //Change the instructor
                system.debug(sDay.Course_Name__c);
                eve.Description='Instructor Name :'+ ' ' +sDay.Dummy_Instructor_Field__c+ '' + '\t\n' +''+ 'Session Name :'+ ' ' +sDay.Name+ '' + '\t\n' +''+ 'Course Name :'+ ' ' +sDay.Courses_Names__c+'' + '\t\n' +''+ 'Location :' + ' ' +sDay.Facility__c+''+ '\t\n' +''+ 'Is Catering Required ? :'+ ' ' +sDay.Is_Catering_Required__c+'' + '\t\n' + '\t\n' +''+ 'Session URL :' + ' ' +URL.getSalesforceBaseUrl().toExternalForm() + '/' + sDay.Id+'' + '\t\n' +''+ 'Additional Requests :'+ '\t\n' +sDay.Additional_Request__c;
                eve.Location=sDay.Facility__c;
               if(eve.ID!=null){ //if event id is not null
                  system.debug('*****'+eve);
                  eventtoUpdate.add(eve); //add event to the list
              }
                  else{
                    Event ee=new Event(); //create a new event
                    ee.OwnerId=label.Contain_ID_of_the_data_CRM; //copy the logged in user id to event
                    ee.ActivityDateTime=sDay.Start_Date_New__c;
                    ee.Description='Session Event';
                    ee.EndDateTime=sDay.End_Date_New__c;
                    ee.IsReminderSet=True;
                    ee.Location=sDay.Facility__c;
                    ee.IsAllDayEvent=False; //Evnt is not all day
                    ee.Instructor__c=sDay.Dummy_Instructor_Field__c;
                    ee.Session_Name__c=sDay.Name;
                    ee.Session_Status__c='scheduled';
                    ee.Subject=sDay.Name;
                    ee.Description='Instructor Name :'+ ' ' +sDay.Dummy_Instructor_Field__c+ '' + '\t\n' +''+ 'Session Name :'+ ' ' +sDay.Name+ '' + '\t\n' +''+ 'Course Name :'+ ' ' +sDay.Courses_Names__c+'' + '\t\n' +''+ 'Location :' + ' ' +sDay.Facility__c+''+ '\t\n' +''+ 'Is Catering Required ? :'+ ' ' +sDay.Is_Catering_Required__c+'' + '\t\n' + '\t\n' +''+ 'Session URL :' + ' ' +URL.getSalesforceBaseUrl().toExternalForm() + '/' + sDay.Id+'' + '\t\n' +''+ 'Additional Requests :'+ '\t\n' +sDay.Additional_Request__c;
                    ee.Session_ID__c=sDay.id;  //A variable in event to hold the session ID in the event.
                    ee.SessionDayURL__c = URL.getSalesforceBaseUrl().toExternalForm() + '/' + sDay.Id;  //URL of session record
                    eventslist.add(ee); //Add events to the list @declare variable at line 10
                    system.debug(ee.SessionDayURL__c);
                    system.debug(sDay.Dummy_Instructor_Field__c);
                  }
            }
            
           }

              sfdcs=[Select Start_Date_New__c,End_Date_New__c,Start_Date__c,End_Date__c from SFDC_Class__c where id IN :SessionIdSet];

              for(SFDC_Class__c sfdc : sfdcs){
                    if(sfdc.Start_Date_New__c!=null && sfdc.Start_Date__c!=sfdc.Start_Date_New__c ){ //if value are changed
                    sfdc.Start_Date__c=sfdc.Start_Date_New__c.date(); //then copy the new value to old one
                    }
                    if(sfdc.End_Date_New__c!=null && sfdc.End_Date__c!=sfdc.End_Date_New__c ){
                    sfdc.End_Date__c=sfdc.End_Date_New__c.date();
                    }
                    if(sfdc.Start_Date_New__c!=null || sfdc.End_Date_New__c!=null){
                    sessionupdateList.add(sfdc);
                    }
            }

          //if session list is greater than 0 then update list
           if(sessionupdateList.size()>0)
           {
            update sessionupdateList;
           }

           if(!eventslist.isempty()) //If evenlist is not empty
           {
             insert eventslist; //insert all events
           }

            //if event list is greater than 0 then update the list
           if(eventtoUpdate !=null && eventtoUpdate.size()>0)
           {
            system.debug(eventtoUpdate);
            system.debug(eventtoUpdate.size());
            system.debug('in update');
            update eventtoUpdate;
           }
        }
      }
}

Now When i am massupdating my records from list view button this trigger get run and i am getting the below error

Error:
There is an error: Update failed. First exception on row 0 with id a08c00000080wefAAA; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, CreateEvents: execution of AfterUpdate caused by: System.DmlException: Update failed. First exception on row 0 with id 00Uc0000003cwSPEAY; first error: INVALID_ID_FIELD, invalid record id: [] Trigger.CreateEvents: line 208, column 1: []

For troubleshooting purpose i remove all the code in trigger on update context and just add some line which basically test whther i am able to update the event on not so  for testing purpose i added the below line after removing all the codes in update context.

if(trigger.isUpdate && trigger.isAfter){  //trigger work upon the update event
       if(preventTriggerRecursion.updatedOnce1()) //To check whether the trigger is not get stuck in recursion if we are updating the same object
        {

eventList=[Select StartDateTime FROM Event limit 2];
          system.debug(eventList);
          update eventList;


}

Note 1 : When i select multiple records from list view I get an error but when i select one record from lis view it works fine.

Note 2:  My trigger works fine when i am updating multiple records through data loader.

Below is the code to prevent the recursive trigger

 

public class preventTriggerRecursion
{
    private static boolean updatedRec1 = true;

    public static boolean updatedOnce1()
    {
        if(updatedRec1)
        {
            updatedRec1 = false;
            return true;
        }
        else
        {
            return updatedRec1;
        }
    }


    private static boolean updatedRec2 = true;

    public static boolean updatedOnce2()
    {
        if(updatedRec2)
        {
            updatedRec2 = false;
            return true;
        }
        else
        {
            return updatedRec2;
        }
    }

}

I actually could not able to out figure why i am getting this strange error, I can't just able to update the event