I have created a Customer dataobject by extending Member, also created many_many data-relation with Package dataobject as mention, I would like increment/decrement 'Credits' field in Customer dataobject, when package is linked/unlinked ie based on Package.Limit

       <?php   
		class Customer extends Member {

		  private static $db = array(
		    'Gender' => 'Varchar(2)',
		    'DateOfBirth' => 'Date',
		    'Featured' => 'Boolean',
		    'Credits' => 'Int'
		  ); 
	
		  private static $many_many = array(
		    "Packages" => "Package",
		  );

		  
	
			public function getCMSFields() {
				//$fields->addFieldToTab('Root.Main',new TextField('Team',"Team"));
				$fields = new FieldList();

		
			// Create a default configuration for the new GridField, allowing record editing
			$config = GridFieldConfig_RelationEditor::create();
				$config->removeComponentsByType('GridFieldAddNewButton');
			// Create a gridfield to hold the package
			$packageField = new GridField(
			    'Packages', // Field name
			    'Package', // Field title
			    $this->Packages(), // List of all related students
			    $config
			);        
			// Create a tab named "Package" and add our field to it
			$fields->addFieldToTab('Root.Package', $packageField); 
				
			Session::set("SingleID", $this->ID);
			//echo $this->ID;
		
				$this->extend('updateCMSFields', $fields);

				return $fields;

			}
	
	
		}

		class Package extends DataObject{
	
			private static $db = array(
					'Title' => 'Varchar(255)',
					'Limit' => 'Int',
				);
	
			private static $belongs_many_many = array(
				"Customers" => "Customer"
			);	
	
	
		}

i tried the below code, but the onAfterWrite event does seems to work.

class  Customer_Packages extends DataObject{

	private static $db = array(
	);
	
	private static $has_one = array(
		"Customer" => "Customer",
		"Package" => "Package"
	);
	

    public function onAfterWrite(){	
		parent::onAfterWrite();	
		if($this->CustomerID){			
			$customer = Customer::get()->filter(array(
				'ID' => $this->CustomerID
				));
			$customer->Credits += 10;
			$customer->write();			 
		}		
		$fd = fopen(dirname(__FILE__).'/test.txt', "w");			
		fwrite($fd, print_r($this,true));
		fclose($fd);				
		
	}		
	
}