trigger count_attachements on Attachment (after insert, after update, after delete, after undelete) {
    Set<Id> parentIdsSet = new Set<Id>(); //stores the parentids linked with modified attachments
    List<Attachment> attachmentList = new List<Attachment>(); //stores the list of modified attachments

    if (Trigger.new != null) {
        attachmentList.addAll(Trigger.new);
    } else if (Trigger.old != null){
        attachmentList.addAll(Trigger.old);
    }

    for (Attachment at: attachmentList) {
        parentIdsSet.add(at.ParentId);
    }

    parentIdsSet.remove(null);

    //stores mapping parentIds to total attachment count for that id
    Map<Id, Integer> objecttIdToAttachmentCount = new Map<Id, Integer>();

    for(AggregateResult ar: [SELECT count(Id) attachmentCount, parentId FROM Attachment WHERE parentId IN :parentIdsSet GROUP BY parentId]) {
        Id objId = (Id)ar.get('parentId');
        Integer count = (Integer)ar.get('attachmentCount');

        count = count == null ? 0 : count;
        objecttIdToAttachmentCount.put(objId, count);
    }

    //for cases where parentid's last attachment is deleted as the above SOQL query wont include the deleted attachments
    for(Id parentId: parentIdsSet) {
        if(!objecttIdToAttachmentCount.containsKey(parentId)) {
            objecttIdToAttachmentCount.put(parentId, 0);
        }
    }

    // not necessary but useful if multiple object types in the parent list
    Map<String, String> sobjectNameToTargetField = new Map<String, String> {
        'Assignement__c' => 'Count_Attachment__c'
    };

    List<SObject> parentRecordsToUpdate = new List<SObject>();

    for (Id parentId: objecttIdToAttachmentCount.keySet()) {
        SObjectType currentType = parentId.getSObjectType();

        if(!sobjectNameToTargetField.containsKey(currentType.getDescribe().getName())) {
            continue;
        }

        SObject parent = currentType.newSObject();
        parent.put(sobjectNameToTargetField.get(currentType.getDescribe().getName()), objecttIdToAttachmentCount.get(parentId));

        parentRecordsToUpdate.add(parent);
    }

    parentRecordsToUpdate.sort();
    update parentRecordsToUpdate;

}