MongoDB Aggregation Count
Example
How do you get the number of Debit and Credit transactions? One way to do it is by using count() function as below.
> db.transactions.count({cr_dr : "D"});
or
> db.transactions.find({cr_dr : "D"}).length();
But what if you do not know the possible values of cr_dr upfront. Here Aggregation framework comes to play. See the below Aggregate query.
> db.transactions.aggregate(
[
{
$group : {
_id : '$cr_dr', // group by type of transaction
// Add 1 for each document to the count for this type of transaction
count : {$sum : 1}
}
}
]
);
And the result is
{
"_id" : "C",
"count" : 3
}
{
"_id" : "D",
"count" : 5
}
