MongoDb How to aggregate by month and year

Question

I am new to mongodb and trying to learn Mongodb queries

    {
    "_id" : ObjectId("59815d4704ca1760a45957ca"),
    "userEmail" : "[email protected]",
    "expenseAmount" : 200,
    "expenseRemark" : "aa",
    "expenseCategory" : "billing",
    "entryTime" : ISODate("2017-08-02T05:03:57Z"),
    "__v" : 0
}
{
    "_id" : ObjectId("59815d5404ca1760a45957cb"),
    "userEmail" : "[email protected]",
    "expenseAmount" : 300,
    "expenseRemark" : "ff",
    "expenseCategory" : "transport",
    "entryTime" : ISODate("2017-08-02T05:04:11Z"),
    "__v" : 0
}
{
    "_id" : ObjectId("5980191d04ca1760a45957cd"),
    "userEmail" : "[email protected]",
    "expenseAmount" : 100,
    "expenseRemark" : "rr",
    "expenseCategory" : "billing",
    "entryTime" : ISODate("2017-08-01T06:00:46Z"),
    "__v" : 0
}
{
    "_id" : ObjectId("5980192604ca1760a45957ce"),
    "userEmail" : "[email protected]",
    "expenseAmount" : 200,
    "expenseRemark" : "qq",
    "expenseCategory" : "transport",
    "entryTime" : ISODate("2017-08-01T06:01:03Z"),
    "__v" : 0
}
{
    "_id" : ObjectId("5980192e04ca1760a45957cf"),
    "userEmail" : "[email protected]",
    "expenseAmount" : 470,
    "expenseRemark" : "ff",
    "expenseCategory" : "transport",
    "entryTime" : ISODate("2017-08-01T06:01:11Z"),
    "__v" : 0
}
{
    "_id" : ObjectId("59816ac004ca1760a45957d0"),
    "userEmail" : "[email protected]",
    "expenseAmount" : 500,
    "expenseRemark" : "raj",
    "expenseCategory" : "transport",
    "entryTime" : ISODate("2017-08-02T06:01:26Z"),
    "__v" : 0
}
{
    "_id" : ObjectId("59816acb04ca1760a45957d1"),
    "userEmail" : "[email protected]",
    "expenseAmount" : 100,
    "expenseRemark" : "pet",
    "expenseCategory" : "pets",
    "entryTime" : ISODate("2017-08-02T06:01:37Z"),
    "__v" : 0
}
{
    "_id" : ObjectId("597d7a9c04ca1760a45957d2"),
    "userEmail" : "[email protected]",
    "expenseAmount" : 500,
    "expenseRemark" : "gt",
    "expenseCategory" : "sports",
    "entryTime" : ISODate("2017-07-30T06:20:04Z"),
    "__v" : 0
}
{
    "_id" : ObjectId("597d7aaa04ca1760a45957d3"),
    "userEmail" : "[email protected]",
    "expenseAmount" : 560,
    "expenseRemark" : "mov",
    "expenseCategory" : "entertainment",
    "entryTime" : ISODate("2017-07-30T06:20:14Z"),
    "__v" : 0
}

I want to get expenseAmount grouped by Year + Month. I tried

`db.expenses.aggregate( 
       {$project : { 
              month : {$month : "$entryTime"}, 
              year : {$year :  "$entryTime"}
          }}, 
        {$group : { 
                _id : {month : "$month" ,year : "$year" },  
              total : {$sum : "$expenseAmount"} 
        }})`

Which gives

{ "_id" : { "month" : 7, "year" : 2017 }, "total" : 0 }

{ "_id" : { "month" : 8, "year" : 2017 }, "total" : 0 }

Please guide me how can I get aggregated result. I could not figure out the way to do that.

Thanks

Bulkhead

Because $project only returns the fields you tell it to. Instead you "should" write the whole thing in $group. Much more efficient that way. – Osteoclast

Answer 1

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

Paction

how can i use $addFields, please guide. – Bulkhead

@Bulkhead in this case you don't need it, because you need only expenseAmount field from input document. But if you would need other fields from input document as well, then instead including them all manually via $project operator, you can use $addFields (link in answer is clickable) to specify only fields which you want to add to existing fields of document – Paction

Answer 2

  db.expenses.aggregate( 
    {$group : { 
            _id : { 
          month : {$month : "$entryTime"}, 
          year : {$year :  "$entryTime"}
      }},  
          total : {$sum : "$expenseAmount"} 
    }})

No need to add "project", only group will do everything.

Iveson

Answer 3

There wont be any need for $project in this case.Just use the below and you would get the expected result.

 db.expenses.aggregate( 
   {$group : { 
           _id : { 
         month : {$month : "$entryTime"}, 
         year : {$year :  "$entryTime"}
     }},  
         total : {$sum : "$expenseAmount"} 
   }})

Monmouthshire