suppose the document below:

{{}}

//  
{
     itemList:[
		{id:20,num:12}，
		{id:1,num:10}，
		{id:2,num:12}，
		{id:12,num:12}]，
     flowerId:0
}

{{}}

how did i update the element with id =1 to make its num decrease 2 and the element with id =2 to make its num decrease 3 in a single statement?