Can you Help me understand what I have done wrong with this trigger?

Question

The trigger should look at all contacts on an account and find the median NPS Score and update the field on the account called Median NPS score.

trigger NPSScore on Contact (before insert) {
     for (Contact contact : trigger.new){
        Set<Id> ContactId = new Set<Id>();
        List<Integer> contactvalues = new List<Integer>();
        for(Contact c:[Select Id, NPS_Rating__c from Contact where NPS_Rating__c != null ])

Integer sizeOfList = contactvalues.size();
Integer index = sizeOfList - 1;
Decimal median = 0.0;

// sort the list first
contactvalues.sort();

//Calculate median
if (Math.mod(sizeOfList, 2) == 0) {
   median = (contactValues[(index-1)/2] + contactValues[(index/2)+1])/2;
}else{
   median = contactvalues[(index+1)/2];
}

system.debug('the median is: '+median);
        account.Median_NPS_Score__c = median;
     }
}

Answer 1

You can try the code below,

`trigger NPSScore on Contact (after insert, after update) {
    //find out Contact list updated or inserted
    Set ContactIds = new Set();

    for (Contact con : Trigger.new) {
        if(con.NPS_Rating__c != null) {
            ContactIds.add(con.Id);
        }
    } 


//find out the parent Account list of Contact
Set<Id> AccountIds = new Set<Id>();
for(Contact con:[Select AccountId from Contact Where AccountId != null And Id IN:ContactIds ]) {
    AccountIds.add(con.AccountId);
}

//for each Account, calculate Median value and insert to the Account
List<Account> accountforupdate = new List<Account>();
for(Account acc:[Select Id, (Select NPS_Rating__c from Contacts where NPS_Rating__c != null ) from Account Where Id IN:AccountIds]) {
    List<Integer> contactvalues = new List<Integer>();
    for(Contact con:acc.Contacts) {
        contactvalues.add(Integer.valueOf(con.NPS_Rating__c));
    }

    Integer sizeOfList = contactvalues.size();
    Integer index = sizeOfList - 1;
    Decimal median = 0.0;

    // sort the list first
    contactvalues.sort();
    //Calculate median
    if (Math.mod(sizeOfList, 2) == 0) {
       median = (contactValues[(index-1)/2] + contactValues[(index/2)+1])/2;
    }else{
       median = contactvalues[(index+1)/2];
    }
    system.debug('the median is: '+median);
    acc.Median_NPS_Score__c = median;

    accountforupdate.add(acc);                  
}
update accountforupdate;   

}`