如何统计mongodb聚合中boolean类型的字段？

Question

我通过 $regex 使用 $lookup 聚合了阶段。在此代码中：

我有团体和学生收藏。

Groups: {_id : ObjectId, title: String}
Students{mainGroups: String,isReserved: Boolean}

mainGroups 是一个连接字符串，对于所有组，每个学生都在学校参加过。

我的代码为我提供了所有组的所有学生集合，无论有多少学生都被返回。

    [
      {
          $addFields: {
              gid: { $toString: "$_id" }
          }
      },
      {
        $lookup: {
            from: 'students',
            let: { groupId: "$gid" },
            pipeline: [
                {
                    $match: {
                        $expr: {
                            $regexMatch: {
                                input: "$mainGroups",
                                regex: { $concat: [".*", "$$groupId", ".*"] },
                                options: "i"
                            }
                        }
                    }
                }
            ],
            as: "students"
        }
      },
      {
          $project: {
              _id: 1,
              gid: 1,
              title: 1,
              studentCount: { $size: "$students" }
          }
      }
    ]

我想通过学生集合中的 $students.isReserved 布尔属性获取totalIsReservedStudents字段。

我尝试通过 $group 获取：

[
  {
      $addFields: {
          gid: { $toString: "$_id" }
      }
  },
  {
    $lookup: {
        from: 'students',
        let: { groupId: "$gid" },
        pipeline: [
            {
                $match: {
                    $expr: {
                        $regexMatch: {
                            input: "$mainGroups",
                            regex: { $concat: [".*", "$$groupId", ".*"] },
                            options: "i"
                        }
                    }
                }
            }
        ],
        as: "students"
    }
  },
  {
    $addFields: {
      studentCount: { $size: "$students" }
    }
  },
  {
    $group: {
      _id: "$students._id",
      totalStudents: {
        $sum: 1
      },
      totalStudentsReserved: {
        $sum: {
          $cond: {
            if: {
              $eq: [
                "$isReserved",
                true
              ]
            },
            then: 1,
            else: 0
          }
        }
      },
    }
  },
  {
    $project: {
        _id: 1,
        gid: 1,
        title: 1,
        studentCount: 1,
        totalStudents:1,
        totalStudentsReserved:1
    }
  },
]

如何获得预订学生以及团体和学生收藏的总数？

Answer 1

一种选择是在

$lookup

管道内进行分组：

db.groups.aggregate([
  {
    $addFields: {
      gid: {
        $toString: "$_id"
      }
    }
  },
  {
    $lookup: {
      from: "students",
      let: {
        groupId: "$gid"
      },
      pipeline: [
        {
          $match: {
            $expr: {
              $regexMatch: {
                input: "$mainGroups",
                regex: {
                  $concat: [
                    ".*",
                    "$$groupId",
                    ".*"
                  ]
                },
                options: "i"
              }
            }
          }
        },
        {
          $group: {
            _id: null,
            count: {
              $sum: 1
            },
            isReserved: {
              $sum: {
                $cond: [
                  "$isReserved",
                  1,
                  0
                ]
              }
            }
          }
        }
      ],
      as: "students"
    }
  },
  {
    $project: {
      _id: 1,
      gid: 1,
      title: 1,
      studentCount: {
        $ifNull: [
          {
            $first: "$students.count"
          },
          0
        ]
      },
      isReserved: {
        $ifNull: [
          {
            $first: "$students.isReserved"
          },
          0
        ]
      }
    }
  }
])

查看它在 playground 示例中的工作原理