MongoError：$subtract 累加​​器是一元运算符

Question

我似乎找不到任何解决此问题的方法。我就是不知道哪里出了问题。

我已经尝试过Mongo subtract in group aggregation，但没有希望。该帖子也未能进一步解释。

这是我的代码：

 var aggregateOptions;
  aggregateOptions = [{
    $match: {

    }
  },{
    $group: {
      _id: "$customerID",
      totalAmount: { $sum: "$amount" },
      totalpaidAmount: { $sum: "$paidAmount" },
      totalAmountDue: { $subtract: ["$totalAmount", "totalpaidAmount"] }
    }
  }];

  Sale.sales.get(Sale.Schema, aggregateOptions, (err, saleRecord) => {
    if (err) throw err;
    console.log(saleRecord);

    res.json({
      pageTitle: "Customer List",
      currentUser: req.user,
      saleRecord: saleRecord,
    });
  });

每次查询都会提示MongoError: The $subtract accumulator is a unary operator。

Answer 1

将您的查询部分更改为此：

aggregateOptions = [
  {
     $match: {

     }
  },
  {
     $group: {
       _id: "$customerID",
       totalAmount: { $sum: "$amount" },
       totalpaidAmount: { $sum: "$paidAmount" }
     }
  },
  {
     $addFields:{
       totalAmountDue: { $subtract: ["$totalAmount", "$totalpaidAmount"] }
     }
  }
];