Insert Trigger on Account(Using MAP)

Question

In my trigger,I would try to update contact description field when i insert a new account and add some value in description field,But when i save the account i will face this error.

trigger updatecontactusingmap on Contact (before insert, before update) {
  Map<Id,Contact> mp = new Map<Id,Contact>();  // new map 

  for(Contact con :trigger.new){  
     mp.put(con.AccountId,con);  // enter contact id in map
  }
  List<Account> listacc = [SELECT Id,Description FROM Account WHERE Id IN:mp.KeySet()];  //  fetch the account details

  Map<Id,Account> mp1 = new Map<Id,Account>();  // new map 
  for(Account a : listacc){  //  enter the list details in a
     mp1.put(a.Id,a);   //   enter account id in map
  }
  for(Contact con :trigger.new){   //new contacts 
     con.Description = mp1.get(con.AccountId).Description;  //  discription field 
  }
}

first error:

CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, updatecontactusingmap: execution of BeforeInsert caused by: System.NullPointerException: Attempt to de-reference a null object Trigger

Answer 1

There are a lot of improvements that need to be done on your code.
1.

  Map<Id,Contact> mp = new Map<Id,Contact>();  // new map 
  for(Contact con :trigger.new){  
     mp.put(con.AccountId,con);  // enter contact id in map
  }

Here you need not create a map if you are using the map.keySet() method in the SOQL. Instead create a Set<Id> and use it in the SOQL.
2.

  Map<Id,Account> mp1 = new Map<Id,Account>();  // new map 
  for(Account a : listacc){  //  enter the list details in a
     mp1.put(a.Id,a);   //   enter account id in map
  }

Here as you are fetching the listacc from SOQl, you can directly create a Map<Id, Account> from it. Use:

Map<Id, Account> mapIdAccount = new Map<Id, Account>([YOUR SOQL QUERY]);

When you pass a list of sObjects as a parameter to the map constructor, it creates a map of Id and the sObject. Refer Salesforce Map Documentation
3.

  for(Contact con :trigger.new){   //new contacts 
     con.Description = mp1.get(con.AccountId).Description;  //  discription field 
  }

Here before you set con.Description, check whether mp1.get is null or not. If the map doesn't contain that key, the get() method simply returns null. Hence it produces a NullPointerException.
Use

if(mp1.containsKey(con.AccountId))
   con.Description = mp1.get(con.AccountId).Description;

Answer 2

I don't think you have to use Trigger in this case, and adding a custom formula field instead of Contact.Description field which is lookup to Account.Description is a more simple solution.

If you really want to use Trigger,you will need both of Contact Trigger and Account Triger to keep the same values between Contact and Account.

And if you only want to fix the CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY error,how about the following code.

trigger updatecontactusingmap on Contact (before insert, before update) {
  Map<Id,Contact> mp = new Map<Id,Contact>();  // new map 

  for(Contact con :trigger.new){  
    if(String.isEmpty(con.AccountId)){
        continue;
    }
     mp.put(con.AccountId,con);  // enter contact id in map
    }
  }
  Set<Id> idSet = mp.KeySet();
  if(!idSet.isEmpty()){
    Map<Id,Account> mp1 = new Map<Id,Account>([SELECT Id,Description FROM Account WHERE Id IN:idSet]);  //  fetch the account details
      for(Contact con :trigger.new){   //new contacts 
        if(String.isEmpty(con.AccountId)){
            continue;
        }
        con.Description = mp1.get(con.AccountId).Description;  //  discription field 
      }
  }

}

Answer 3

Try this one it will work

trigger updateContactDescription on Contact (before insert,before update) {

   Map<Id,contact> map1 = new Map<Id,contact>();
   for(contact con : trigger.new)
     map1.put(con.AccountId,con);

   //if(map1.size() > 0)
   Map<Id,Account> map2 = new Map<Id,Account>([SELECT Id ,Description FROM Account WHERE Id IN :map1.keySet()]);

   for(Contact con : Trigger.new){
      if(map2.get(con.AccountId).Description != NULL)
         con.Description = map2.get(con.AccountId).Description;
   }
}