MongoDb How to aggregate by month and year

You have lost expenseAmount field during projection stage. Simply add it:

   {$project : { 
          month : {$month : "$entryTime"}, 
          year : {$year :  "$entryTime"},
          expenseAmount : 1
      }},

Note that if field does not exist in document, then $sum returns 0.

Also note that there is another aggregation operator which performs as you expected - $addFields. It adds new fields to document and preserves all existing fields from the input document. But in this case you need only expenseAmount field