Mouse position relative to div

I am using jquery ui for drag and drop. I am trying to get mouse position relative to div, here is my code:

$( "#db_tables " ).droppable({
  activeClass: "ui-state-default",
  hoverClass: "ui-state-hover",
  drop: function( event, ui ) {
    var x = ui.position.left - ui.offset.left; // tired event.pageX - this.offsetLeft;
    var y = ui.position.top - ui.offset.top; // tired event.pageY - this.offsetTop;
    $( '<div style="margin-top:' + y   + 'px; margin-left:' + x   + 'px; "></div>' ).html( ui.draggable.html() ).appendTo( this );
  }
});

But the position of dropped div is not correct, Can anybody please tell me what is wrong with code?