db.aa.aggregate([                                          { $group: { _id: null, total: { $sum: "$value" } } }, //$value 指的是按照那一列来sum。比如 value是mongo的一列。那么此处应该是 $value                     { $sort: { total: -1 } }                   ])

db.myCollection.find( { $where: "this.credits == this.debits" } );db.myCollection.find( { $where: "obj.credits == obj.debits" } );db.myCollection.find( { $where: function() { return (this.credits == this.debits) } } );db.myCollection.find( { $where: function() { return obj.credits == obj.debits; } } );