MongoDB Aggregate

{
    "_id" : ObjectId("50b1aa983b3d0043b51b2c52"),
    "name" : "Nexus 7",
    "category" : "Tablets",
    "manufacturer" : "Google",
    "price" : 199
}

db.products.aggregate([
    {$group:
     {
	 _id:"$manufacturer", 
	 num_products:{$sum:1}
     }
    }
])

{ "_id" : "Amazon", "num_products" : 2 }
{ "_id" : "Sony", "num_products" : 1 }
{ "_id" : "Samsung", "num_products" : 2
{ "_id" : "Google", "num_products" : 1 }
{ "_id" : "Apple", "num_products" : 4 }