Update Number Of Child Contacts on Account for Contact changes

 he UpdateContactCountOnAccount trigger ensures that the custom field Number_Of_Child_Contacts__c on the parent Account is updated with the correct count of associated Contacts. It runs after insert, after update, after delete, and after undelete on the Contact object. The trigger collects Account IDs for Contacts being processed, queries all related Contacts, manually counts them, and updates the custom field on the corresponding Accounts. 

trigger UpdateContactCountOnAccount on Contact (after insert, after update, after delete, after undelete) {

    Set<Id> accountIdsToUpdate = new Set<Id>();

    if (Trigger.isInsert || Trigger.isUndelete) {

        for (Contact c : Trigger.new) {

            accountIdsToUpdate.add(c.AccountId);

        }

    } else if (Trigger.isUpdate) {

        for (Contact c : Trigger.new) {

            accountIdsToUpdate.add(c.AccountId);

        }

        for (Contact c : Trigger.old) {

            accountIdsToUpdate.add(c.AccountId);

        }

    } else if (Trigger.isDelete) {

        for (Contact c : Trigger.old) {

            accountIdsToUpdate.add(c.AccountId);

        }

    }

    // Map to hold accountId and corresponding contact count

    Map<Id, Integer> accountContactCountMap = new Map<Id, Integer>();

    // Initialize the map with account IDs

    for (Id accountId : accountIdsToUpdate) {

        accountContactCountMap.put(accountId, 0);

    }

    // Query all contacts for the accounts to be updated

    List<Contact> contacts = [

        SELECT AccountId 

        FROM Contact 

        WHERE AccountId IN :accountIdsToUpdate

    ];

    // Count the number of contacts for each account

    for (Contact contact : contacts) {

        if (accountContactCountMap.containsKey(contact.AccountId)) {

            accountContactCountMap.put(contact.AccountId, accountContactCountMap.get(contact.AccountId) + 1);

        } else {

            accountContactCountMap.put(contact.AccountId, 1);

        }

    }

    // Prepare the list of accounts to be updated

    List<Account> accountsToUpdate = new List<Account>();

    for (Id accountId : accountContactCountMap.keySet()) {

        accountsToUpdate.add(new Account(Id = accountId, Number_Of_Child_Contacts__c = accountContactCountMap.get(accountId)));

    }

    // Update the accounts

    if (!accountsToUpdate.isEmpty()) {

        update accountsToUpdate;

    }

}