在 Mongodb/Mongoose 聚合中使用 $lookup 和 $group答案

【问题标题】：Using a $lookup with $group in Mongo/Mongoose Aggtregation在 Mongodb/Mongoose 聚合中使用 $lookup 和 $group
【发布时间】：2019-01-21 07:25:33
【问题描述】：

我们设置了一些包装请求函数来规范我们处理请求的方式。我们设置的函数之一是聚合函数。它看起来像这样：

  async aggregate(mongooseModelObject, aggregateObject = {}) {
    try {
      return await mongooseModelObject.aggregate(aggregateObject).exec();
    } catch (err) {
      this.sendError(err);
    }
  }

当我这样使用它时效果很好：

exports.getCountByBranch = async function (req, res) {
  let docs;
  let request = new EndpointRequestController(req, res);

  try {
    docs = await request.aggregate(staffmember, [{
      $group: {
        _id: "$branch",
        count: {
          $sum: 1
        }
      }
    }]);
  } catch (err) {
    return request.sendError("An error occurred while trying to find existing records.", err);
  }
  request.documentCount = docs.length;
  request.sendResponse(docs);
}

但是，为了让最终用户对用户更加友好，我想做的是运行 populate 以包含分支“名称”，而不仅仅是“_id”。所以我认为这就像使用$lookup 添加另一个阶段一样简单。如果我只是直接调用 mongoose 函数，我会这样做。也就是说，我试过这个：

exports.getCountByBranch = async function (req, res) {
  let docs;
  let request = new EndpointRequestController(req, res);

  try {
    docs = await request.aggregate(staffmember, [{
      $lookup: {
        from: "branches",
        localField: "branch",
        foreignField: "_id",
        as: "branch"
      },
      $group: {
        _id: "$branch",
        count: {
          $sum: 1
        }
      }
    }]);
  } catch (err) {
    return request.sendError("An error occurred while trying to find existing records.", err);
  }
  request.documentCount = docs.length;
  request.sendResponse(docs);
}

但这会出错：

错误：参数必须是聚合管道运算符

我在这里错过了什么？

【问题讨论】：

试试.allowDiskUse(true)
使用这个 [{ $lookup: { from: "branches", localField: "branch", foreignField: "_id", as: "branch" }}, {$group: { _id: " $branch", count: { $sum: 1 } } }] 花括号在管道阶段没有正确闭合
就是这样，谢谢@SenthurDeva。

标签： node.js mongodb mongoose aggregation-framework

【解决方案1】：

试试这个效果很好

exports.getCountByBranch = async function (req, res) {
let docs;
let request = new EndpointRequestController(req, res);

try {
    docs = await request.aggregate(staffmember, [{
        $lookup: {
            from: "branches",
            localField: "branch",
            foreignField: "_id",
            as: "branch"
        }
    },
    {
        $group: {
            _id: "$branch",
            count: {
                $sum: 1
            }
        }
    }]);
} catch (err) {
    return request.sendError("An error occurred while trying to find existing records.", err);
}
request.documentCount = docs.length;
request.sendResponse(docs);

}

【讨论】：