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} }}]);

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