AccountRegionTrigger, Handler and Test class

AccountRegionTriggerHandler

public with sharing class AccountRegionTriggerHandler {
  
  @future 
  public static void ProcessRegionsAsync(Set<ID> accountIds){
    
    // holds a map of the account id and comma separated regions to build
    Map<Id, String> accountRegionMap = new Map<Id, String>();
      
    // get ALL of the regions for all affected accounts so we can build
    List<Account_Region__c> accountRegions = [select id, Account__c, 
      Region__r.Name from Account_Region__c 
      where Account__c IN :accountIds order by Region__r.Name];
      
    for (Account_Region__c ar : accountRegions) {
      if (!accountRegionMap.containsKey(ar.Account__c)) {
        // if the key (account) doesn't exist, add it with region name
        accountRegionMap.put(ar.Account__c,ar.Region__r.Name);
      } else {
        // if the key (account) already exist, add ", region-name"
        accountRegionMap.put(ar.Account__c,accountRegionMap.get(ar.Account__c) + 
          ', ' + ar.Region__r.Name);
      }
    }
    
    // get the account that were affected
    List<Account> accounts = [select id from Account where Id IN :accountIds];
    
    // add the comma separated list of regions
    for (Account a : accounts)
      a.Regions__c = accountRegionMap.get(a.id);
    
    // update the accounts
    update accounts;
    
  }  
  
}

gistfile1.tex

trigger AccountRegionTrigger on Account_Region__c (after delete, after insert, after update) {
      
  // fires after both insert and update
  if((Trigger.isInsert || Trigger.isUpdate) && Trigger.isAfter){
    
    // find the ids of all accounts that were affected
    Set<Id> accountIds = new Set<Id>();
    for (Account_Region__c ar : [select Id, Account__c from Account_Region__c 
      where Id IN :Trigger.newMap.keySet()])
      accountIds.add(ar.Account__c);
        
    // process the accounts  
    AccountRegionTriggerHandler.ProcessRegionsAsync(accountIds);
    

  // fires when records are deleted. may want to do undelete also?
  } else if(Trigger.isDelete && Trigger.isAfter){
    
    // find the ids of all accounts that were affected
    Set<Id> accountIds = new Set<Id>();
    for (ID id : Trigger.oldMap.keySet())
      accountIds.add(Trigger.oldMap.get(id).Account__c);
    
    // process the accounts
    AccountRegionTriggerHandler.ProcessRegionsAsync(accountIds);

  }

}

Test_AccountRegionTriggerHandler

@isTest
private class Test_AccountRegionTriggerHandler {
  
  static List<Region__c> regions = new List<Region__c>();
  
  static {
  
    // insert some regions
    Region__c r1 = new Region__c(name='Region 1');
    Region__c r2 = new Region__c(name='Region 2');
    Region__c r3 = new Region__c(name='Region 3');
    Region__c r4 = new Region__c(name='Region 4');
    regions.add(r1);
    regions.add(r2);
    regions.add(r3);
    regions.add(r4);
    insert regions;
    
  }
  
  private static void testInsertRecords() {
    
    List<Account> accounts = new List<Account>();
    List<Account_Region__c> accountRegions = new List<Account_Region__c>();
    
    // insert some accounts
    Account a1 = new Account(name='Account 1');
    Account a2 = new Account(name='Account 2');
    accounts.add(a1);
    accounts.add(a2);
    insert accounts;
    
    Test.startTest();
    
      accountRegions.add(new Account_Region__c(Account__c=a1.Id, Region__c=regions.get(0).Id));
      accountRegions.add(new Account_Region__c(Account__c=a1.Id, Region__c=regions.get(1).Id));
      accountRegions.add(new Account_Region__c(Account__c=a2.Id, Region__c=regions.get(2).Id));
      accountRegions.add(new Account_Region__c(Account__c=a2.Id, Region__c=regions.get(3).Id));
      
      insert accountRegions;
    
    Test.stopTest();
    
    // since async, check for the accounts AFTER tests stop
    List<Account> updatedAccounts = [select id, name, regions__c from account where id IN :accounts];
    System.assertEquals('Region 1, Region 3',updatedAccounts.get(0).Regions__c);
    System.assertEquals('Region 2, Region 4',updatedAccounts.get(1).Regions__c);
    
  }
  
  private static void testDeleteRecords() {
    
    List<Account> accounts = new List<Account>();
    List<Account_Region__c> accountRegions = new List<Account_Region__c>();
    
    // insert an account
    Account a1 = new Account(name='Account 1');
    accounts.add(a1);
    insert accounts;
    
    Test.startTest();
    
      accountRegions.add(new Account_Region__c(Account__c=a1.Id, Region__c=regions.get(0).Id)); 
      accountRegions.add(new Account_Region__c(Account__c=a1.Id, Region__c=regions.get(1).Id));
      accountRegions.add(new Account_Region__c(Account__c=a1.Id, Region__c=regions.get(2).Id));
      accountRegions.add(new Account_Region__c(Account__c=a1.Id, Region__c=regions.get(3).Id));
      
      insert accountRegions;
    
      // now delete a record
      delete accountRegions.get(3);
    
    Test.stopTest();
    
    List<Account> updatedAccounts = [select id, name, regions__c from account where id IN :accounts];
    System.assertEquals('Region 1, Region 2, Region 3',updatedAccounts.get(0).Regions__c);
    
  }
  
}