{
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} }
}
]);