$ lookup后下一个聚合阶段丢失数据

Question

感谢来到我的岗位。我是MongoDB的新手，我需要帮助。我需要从数据库中选择用户数据，所以我有一个非常简单的管道：

const pipeline = [{
    $match: {
      type: 'student',
      counselorUserName: username
    },
    $project: {
      username: '$username',
      email: '$email',
      phone: '$phone',
      fullName: '$fullName',
      gradeLevel: {
        $switch: {
          branches: [{
              case: {
                $eq: ['$gradeLevel', '9']
              },
              then: 'Freshman'
            },
            {
              case: {
                $eq: ['$gradeLevel', '10']
              },
              then: 'Sophomore'
            },
          ],
          default: "Freshman"
        }
      }
    },
    $lookup: {
      from: 'RoadmapTasksCompleted',
      let: {
        username: '$username',
        gradeLevel: '$gradeLevel'
      },
      pipeline: [
        {
          $match: {
            monthToComplete: {
              $in: prevMonthsNames
            },
            $expr: {
              $and: [
                {
                  $eq: ['$username', '$$username']
                },
                {
                  $eq: ['$gradeLevel', '$$gradeLevel']
                }
              ]
            }
          }
        },
        {
          $group: {
            _id: null,
            count: { $sum: 1 }
          }
        }
      ],
      as: 'completedTasksCount'
    }
}

工作正常，我得到：

{
    username: "hewy",
    email: "htheodros@usfca.edu",
    phone: "4049159553",
    fullName: "Hewotte Theodros",…} 
    completedTasksCount:[{_id: null, count: 27}]
}

但是我需要在查找下面添加新的阶段，当我这样做时，completedTasksCount变成空数组[]，为什么会发生这种情况以及如何解决呢？

我需要添加以下阶段：

    $project: {
      username: '$username',
      email: '$email',
      phone: '$phone',
      fullName: '$fullName',
      completedTasks: { $arrayElemAt: ['$completedTasksCount', 0] },
      tasksCountMap: tasksCountArray
    },
    $addFields: {
      status: {
        $let: {
          vars: {
            tasksCount: {
              $arrayElemAt: [
                "$tasksCountMap.v",
                {
                  $indexOfArray: ["$tasksCountMap.k", "$gradeLevel"]
                }
              ]
            },
            completedTasksCount: '$completedTasks.count'
          },
          in: {
            $cond: {
              if: { $or: [
                { $eq: ['$$tasksCount', '$$completedTasksCount'] },
                { $lte: ['$$tasksCount', '$$completedTasksCount'] },
              ]},
              then: 'On track',
              else: 'High priority'
            }
          }
        }
      }
    },

感谢帮助。