Help with Apex trigger that updates all child Contact records when Account is updated

Question

I am creating my very first Apex trigger and am having trouble with the syntax.

I have a formula field in the Contact object that pulls in the Industry field from the Account. When an account is updated and the Industry is changed, I want the child Contacts to update (no changes to data) so that my "on update" workflow runs (which triggers an outbound message to update my external ERP through Jitterbit).

I have an error: Compile Error: Method does not exist or incorrect signature: [LIST].keySet() at line 6 column 20

I would also like the trigger to fire only when the industry is changed.

My code is below:

trigger UpdateContactsWithNewCustomerType on Account (after update) {

  If(Trigger.isUpdate){

  Set<ID> ids = Trigger.new.keySet();
  List<Account> updatedParents = [SELECT Id, 
     (select ID, AccountID from Contacts) 
      FROM Account 
      WHERE Id in :ids];
  List<Contact> childrenToUpdate = new List<Contact>();
  for (Account p : updatedParents) 
    { 

    for(Contact kid : p.Contacts) 
     {                      
       childrenToUpdate.add(kid);
      }
        }
  if( !childrenToUpdate.isEmpty )
     {
     update childrenToUpdate;
    }
  }  
}

* UPDATE * Thanks to both @Ralph and @Regal I was able to get this working. Below is the final code that worked.

trigger UpdateContactsWithNewCustomerType on Account (after update) {
//CREATE NEW LIST ids THAT CONTAINS ACCOUNT IDS SENT TO TRIGGER
 Set<ID> ids = Trigger.newMap.keySet();

// FIND ALL ACCOUNTS WHERE ID = THOSE SENT TO TRIGGER
List<Account> updatedParents = [SELECT Id, Industry, (select ID, AccountID from Contacts) 
    FROM Account 
    WHERE Id in :ids];

//CREATE NEW EMPTY LIST CALLED childrenToUpdate
List<Contact> childrenToUpdate = new List<Contact>();

//LOOP OVER UPDATED ACCOUNTS
for(Account acc : updatedParents)
 { // CHECK IF THE NEW AND OLD INDUSTRY VALUES ARE THE SAME
  String oldvalue = Trigger.oldMap.get(acc.Id).Industry;
  String newValue = acc.Industry;

 //IF NOT, THEN ADD IDS OF RELATED CONTACTS TO childrenToUpdate LIST
 if(oldvalue != newValue )
  {
    for(Contact kid: acc.Contacts) 
     {                      
       childrenToUpdate.add(kid);
     }

  }
}

//UPDATE CONTACTS IF childrenToUpdate IS NOT EMPTY
if( !childrenToUpdate.isEmpty() )
{ 
  update childrenToUpdate;
 }
}

Answer 1

You should use Trigger.newMap instead of List as explained by Ralph in the answer.

In addition, to check the condition if the industry is changed or not there is an another Trigger context variable Trigger.oldMap where you can get previous value of field and this provide ability to compare the changes in record after update.

Ex:

for(Account acc : Trigger.newMap)
{
     Id oldvalue = Trigger.oldMap.get(acc.Id).Industry_Field__c;
     Id newValue = acc.Industry_Field__c;

     // check if industry field changed or not
     if(oldvalue != newValue )
     {
        // Proceed
     }
}

Answer 2

Trigger.new is a list and you're calling the keySet() method on it, but that method only works for maps. You can use Trigger.newMap instead which does have a keySet() method.