mongo聚合函数查重

根据firstField、secondField字段来查重

db.collection.aggregate([
  { $group: { 
    _id: { firstField: "$firstField", secondField: "$secondField" }, 
    uniqueIds: { $addToSet: "$_id" },
    count: { $sum: 1 } 
  }}, 
  { $match: { 
    count: { $gt: 1 } 
  }}
])