Insert new contact share if account owner changed

Question

I made a trigger to insert new contact share (user) if the account owner is changed.. and I want to insert new contact share without removing all previous contact share (old account owner/contact owner).. When user change the account owner, it will set the new owner for contact ( account owner == contact owner )..

But everytime I changed the account owner, theres no log in dev console about my trigger, and theres only account owner (user) in contact share, all previous contact share is gone..

 trigger AutoShareContact on Contact (before update) {
   List<ContactShare> listContactShare = [SELECT Id, UserOrGroupId, 
                                           ContactAccessLevel, ContactId 
                                          FROM ContactShare];
    List<ContactShare> upCt = new list<ContactShare>();
    for (Contact ct : Trigger.new){
        String accOwnerId = [SELECT Id, OwnerId FROM Account WHERE Id =: 
                             ct.AccountId].OwnerId;

        ContactShare cs = new ContactShare();
        cs.ContactId = ct.id;
        cs.UserOrGroupId = accOwnerId;
        cs.ContactAccessLevel = 'All';
        cs.RowCause = 'Owner';
        upCt.add(cs);
        system.debug('CS ==>'+cs);

        for (ContactShare cShare : listContactShare){
            if (cShare.ContactId == ct.id){
                ContactShare cs2 = new ContactShare();
                cs2.ContactId = ct.id;
                cs2.UserOrGroupId = cShare.UserOrGroupId;
                cs2.ContactAccessLevel = 'Edit';
                cs2.RowCause = 'Manual';
                upCt.add(cs2);
                system.debug('CS2 ==>'+cs2);
            }
        } 
    }
     system.debug('LIST -->'+ upCt);
     insert upCt;
    } // end trigger

Answer 1

You have to write trigger on Account instead of Contact. Since, Account owner field is changed, it won't execute contact trigger. You should write an after update trigger on Account which checks if old value of account owner does not match with new value of account owner, then insert a ContactShare record