trigger updateProductQuantity ON Opportunity(after update)
{
	Set<Id> opIdSet = new Set<Id>();
	Set<Id> productIdSet = new Set<Id>();

	for(Opportunity op : Trigger.new){
		
		if(op.StageName = '100%Closed' && Trigger.newMap.get(op.Id).stageName != '100%Closed') // you give name when you opportunity is closed won and closed
		{
			opIdSet.add(op.Id);
		}
	}

	Map<ProductId,Integer> productToConsumedQuantity = new Map<ProductId,Integer>() ;
	for(OpportunityLineItem oppLI :[SELECT Id, Product2, Quantity FROM OpportunityLineItem WHERE OpportunityId IN: oppLIList] )
	{
		Integer temp ;
		if(productToConsumedQuantity.get(oppLI.Product2) == null )
		{
			productToConsumedQuantity.put(oppLI.Product2,(oppLI.Quantity != null?oppLI.Quantity : 0)) ;
		} else {
			temp = productToConsumedQuantity.get(oppLI.Product2) ;
			temp += oppLI.Quantity != null? oppLI.Quantity : 0 ;
			productToConsumedQuantity.put(oppLI.Product2,temp) ;
		}
		
		productIdSet.add(oppLI.Product2) ;
	}

	List<Product2> prodList = new List<Product2>();
	for(Product2 product : [SELECT Id, CustomQuantityField__c FROM Product2 WHERE Id IN: productIdSet]){

		product.CustomQuantityField__c = product.CustomQuantityField__c - productToConsumedQuantity.get(product.Id) ;
		prodList.add(product) ;
	}

	If(prodList.size() > 0)
	{
		update prodList ;
	}
}