MongoDB Aggregate and Group example

In this tutorial, we will show you how to use MongoDB aggregate function to group documents (data).

1. Test Data

Data in JSON format, shows the hosting provider for website.

{ "_id" : 1, "domainName" : "test1.com", "hosting" : "hostgator.com" }
{ "_id" : 2, "domainName" : "test2.com", "hosting" : "aws.amazon.com"}
{ "_id" : 3, "domainName" : "test3.com", "hosting" : "aws.amazon.com" }
{ "_id" : 4, "domainName" : "test4.com", "hosting" : "hostgator.com" }
{ "_id" : 5, "domainName" : "test5.com", "hosting" : "aws.amazon.com" }
{ "_id" : 6, "domainName" : "test6.com", "hosting" : "cloud.google.com" }
{ "_id" : 7, "domainName" : "test7.com", "hosting" : "aws.amazon.com" }
{ "_id" : 8, "domainName" : "test8.com", "hosting" : "hostgator.com" }
{ "_id" : 9, "domainName" : "test9.com", "hosting" : "cloud.google.com" }
{ "_id" : 10, "domainName" : "test10.com", "hosting" : "godaddy.com" }

Imports into a “website” collection.

> mongoimport -d testdb -c website --file website.json
connected to: 127.0.0.1
Mon Jan 13 14:30:22.662 imported 10 objects

> mongoimport -d testdb -c website --file website.json --upsert

2. Grouping Example

Uses db.collection.aggregate and $group to perform the data grouping.

2.1 The following example groups by the “hosting” field, and display the total sum of each hosting.

> db.website.aggregate(
    { 
	$group : {_id : "$hosting", total : { $sum : 1 }}
  );

Output

        "result" : [
                        "_id" : "godaddy.com",
                        "total" : 1
                },
                        "_id" : "cloud.google.com",
                        "total" : 2
                },
                        "_id" : "aws.amazon.com",
                        "total" : 4
                },
                        "_id" : "hostgator.com",
                        "total" : 3
        ],
        "ok" : 1

The equivalent SQL.

SELECT hosting, SUM(hosting) AS total
       FROM website
       GROUP BY hosting

2.2 Add sorting with $sort.

>  db.website.aggregate(
     { 
	$group : {_id : "$hosting", total : { $sum : 1 }}
     },
	$sort : {total : -1}
  );

Output – Display “total” in descending order. For ascending order, uses $sort : {total : 1}.

        "result" : [
                        "_id" : "aws.amazon.com",
                        "total" : 4
                },
                        "_id" : "hostgator.com",
                        "total" : 3
                },
                        "_id" : "cloud.google.com",
                        "total" : 2
                },
                        "_id" : "godaddy.com",
                        "total" : 1
        ],
        "ok" : 1

2.3 Add $match condition, groups by “hosting” for “aws.amazon.com” only.

> db.website.aggregate(
    { 
	$match : {hosting : "aws.amazon.com"}
    },
    { 
	$group : { _id : "$hosting", total : { $sum : 1 } }
  );

Output

        "result" : [
                        "_id" : "aws.amazon.com",
                        "total" : 4
        ],
        "ok" : 1

3. Exports Grouping Result to CSV or JSON

Often times, we need to export the grouping results in csv or JSON format. To solve it, inserts the group results in a new collection, and exports the new collection via mongoexport.

3.1 Set the group results in a variable. In this case, the variable name is “groupdata”.

> var groupdata = db.website.aggregate(
    { 
	$group : {_id : "$hosting", total : { $sum : 1 }}
    },
	$sort : {total : -1}
  );

3.2Inserts groupdata.toArray() into a new collection.

> db.websitegroup.insert(groupdata.toArray());
> db.websitegroup.find().pretty()
{ "_id" : "aws.amazon.com", "total" : 4 }
{ "_id" : "hostgator.com", "total" : 3 }
{ "_id" : "cloud.google.com", "total" : 2 }
{ "_id" : "godaddy.com", "total" : 1 }
>

3.3 Exports the collection “websitegroup” to a csv file.

c:\> mongoexport -d testdb -c websitegroup -f _id,total -o group.csv --csv
connected to: 127.0.0.1
exported 4 records

_id,total
"aws.amazon.com",4.0
"cloud.google.com",2.0
"godaddy.com",1.0
"hostgator.com",3.0

3.4 Exports the collection “websitegroup” to a JSON file.

c:\> mongoexport -d testdb -c websitegroup -o group.json
connected to: 127.0.0.1
exported 4 records

{ "_id" : "aws.amazon.com", "total" : 4 }
{ "_id" : "cloud.google.com", "total" : 2 }
{ "_id" : "godaddy.com", "total" : 1 }
{ "_id" : "hostgator.com", "total" : 3 }

4. Large Sort Operation

To avoid the sort exceeded memory limit error, enable the allowDiskUse option.

db.website.aggregate(
	{$group : {_id : "$hosting", total : { $sum : 1 }}},
	{$sort : {total : -1}}
],
	{allowDiskUse: true}
);

References

1. MongoDB Aggregation
2. MongoDB db.collection.aggregate()
3. Aggregation Pipeline Limits
4. MongoDB Hello World Example