Count number of Active contact on Account

Count # Number of active contact on Account record.

Scenario: Calculate #NoOfActiveContact for Account record.

Solution: Create an Apex Trigger on Contact object and calculate for all case (insert, update, delete, undelete) 

 trigger CountActiveContact on Contact (after insert,after update,after delete,after undelete) {  
   Set<id> parentId = new Set<Id>(); // AccountId Set  
   if(Trigger.isAfter){  
     if(Trigger.isUpdate || Trigger.isInsert || Trigger.isUndelete){  
       for(Contact con: Trigger.new){  // return list of new sObject record
         if(con.accountId !=null){  //check if accountid is not null
           parentId.add(con.accountId);  //add accountid in set
           if(Trigger.isUpdate){ // check for update   
             if(Trigger.oldMap.get(con.id).AccountId != con.AccountId){ // check old accountid with new  
               parentId.add(Trigger.oldMap.get(con.id).AccountId);//accountid set  
             }  
           }  
         }  
       }  
     }  
     if(Trigger.isDelete){// delete  
       for(Contact con: Trigger.old){  // return list of deleted sObject record
         parentId.add(con.accountId);  
       }  
     }  
     List<Account> accListForUpdate = new List<Account>();  //account list for update
     for(Account acc: [select id,ARI__No_Of_Contact__c,(select id from contacts where isActive__c=true) from Account where id IN: parentId]){  
       acc.ARI__No_Of_Contact__c = acc.contacts.size(); // get size of contact list on account 
       accListForUpdate.add(acc);  //add acc object in list
     }  
     if(accListForUpdate.size()>0){  //check for list size
       try{  
         update accListForUpdate;  // update account list
       }  
       catch(Exception e){  
         system.debug('Exception has occured! '+e.getMessage());  
       }  
     }  
   }  
 }