The reordering method used by the Differential example can lead to data loss
though, because it calculates the new order of a dropped element as the
arithmetic mean of the elements before and after it. This [runs into limitations
of floating point precision](http://programmers.stackexchange.com/questions/266451/maintain-ordered-collection-by-updating-as-few-order-fields-as-possible)
deleteevent.data._id;// Generate a new id when inserting in the Attributes collection. Otherwise, if we add the same type twice, we'll get an error that the ids are not unique.