MongoDB Aggregation: Counting distinct fields

You can use sets

db.test.aggregate([
    {$group: { 
      _id: "$account", 
      uniqueVendors: {$addToSet: "$vendor"}
    }},
    {$project: {
      _id: 1, 
      vendorsCount: {$size: "$uniqueVendors"}
    }}
]);

I figured this out by using the $addToSet and $unwind operators.

Mongodb Aggregation count array/set size

db.collection.aggregate([
{
    $group: { _id: { account: '$account' }, vendors: { $addToSet: '$vendor'} }
},
{
    $unwind:"$vendors"
},
{
    $group: { _id: "$_id", vendorCount: { $sum:1} }
}
]);

Hope it helps someone

I think its better if you execute query like following which will avoid unwind

db.t2.insert({_id:1,account:"abc",vendor:"amazon"});
db.t2.insert({_id:2,account:"abc",vendor:"overstock"});


db.t2.aggregate(
{ $group : { _id : { "account" : "$account", "vendor" : "$vendor" }, number : { $sum : 1 } } },
{ $group : { _id : "$_id.account", number : { $sum : 1 } } }
);

Which will show you following result which is expected.

{ "_id" : "abc", "number" : 2 }