MongoDB aggregate by field exists

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I solved the same problem just last night, this way:

> db.test.aggregate({$group:{_id:{$gt:["$field", null]}, count:{$sum:1}}})
{ "_id" : true, "count" : 2 }
{ "_id" : false, "count" : 2 }

See http://docs.mongodb.org/manual/reference/bson-types/#bson-types-comparison-order for a full explanation of how this works.

Added From comment section:

To check if the value doesn’t exist or is null use { $lte: ["$field", null] }

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