改名重新归类统计

{
 account:"abc",
 vendor:"amazon",
}
 ...
{
 account:"abc",
 vendor:"overstock",
}
想得到
{
 account:"abc",
 vendorCount:2
}

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