HAVING COUNT(column_name) = 1 in mongodb

[Niall Paterson]

Hey guys, 

I'm trying to use HAVING COUNT's equivalent  in mongo.

I've tried: 

    messages.aggregate([ { '$match': { count: 1 } } ])   

Which in SQL would be HAVING COUNT(*) = 1 right?

This returns an empty array. Shouldn't that really return every document in messages?

What I really want to do is have something like HAVING COUNT(column_name) = 1 in mongo.

For the sake of this lets say the column_name is conversation_id.

I've tried this but it doesn't work, (which isn't surprising considering the above doesn't even work):

    messages.aggregate([ { '$match': { count: { conversation_id: 1 }  } } } ])

Nobody seems to have done this online, Does anyone know how do I do this kind of thing in mongo?

Many thanks,

Niall

[Jeff Lee]

Hey Niall,

You want to use $group with $match.

e.g.

> db.foodle.find()

{ "_id" : 1, "conversation_id" : 1 }

{ "_id" : 2, "conversation_id" : 2 }

{ "_id" : 3, "conversation_id" : 3 }

{ "_id" : 4, "conversation_id" : 1 }

{ "_id" : 5, "conversation_id" : 2 }

> db.foodle.aggregate(

... {$group:{_id:{conversationId:"$conversation_id"}, count:{$sum:1}}},

... {$match:{count:1}})

{

        "result" : [

                {

                        "_id" : {

                                "conversationId" : 3

                        },

                        "count" : 1

                }

        ],

        "ok" : 1

}

See the aggregation examples docs for more examples.

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[Niall Paterson]

Amesome, worked perfectly, thanks so much!