MongoDB聚合$graphLookup -在以下关系集合中查找共同的“连接”

Question

我收集了“谁在跟踪谁”(如Instagram)：

db.users.insertMany([
  { _id: 1, name: "Arnold Schwarzenegger" },
  { _id: 2, name: "James Earl Jones" },
  { _id: 3, name: "Harrison Ford" },
  { _id: 4, name: "Jennifer Lawrence" }
]);

db.follows.insertMany([
  { _id: 12, follower: 1, following: 2 },
  { _id: 13, follower: 1, following: 3 },
  { _id: 24, follower: 2, following: 4 },
  { _id: 23, follower: 2, following: 3 }
]);

我试图向其他用户推荐他们可以跟踪的其他用户。例如，他们可以跟踪哪些其他人；建议的追随者，根据现有的共同联系的数量排序。

在本例中：

+--------+--------------+----------+
|   A    | is following |    B     |
+--------+--------------+----------+
| Arnold | ->           | James    |
| Arnold | ->           | Harrison |
| James  | ->           | Jennifer |
| James  | ->           | Harrison |
+--------+--------------+----------+

阿诺德和詹姆斯之间的，谁能跟随阿诺德？(不包括现有的连接)

The answer should be: Jennifer

这是一次失败的尝试：

db.users.aggregate([
  {
    $match: { _id: 1 } // Arnold
  },
  {
    $graphLookup: {
      from: "follows",
      startWith: "$_id",
      connectFromField: "following",
      connectToField: "follower",
      maxDepth: 1,
      as: "connections",
    }
  }
]);

其结果是：

  {
    "_id": 1,
    "name": "Arnold Schwarzenegger",
    "connections": [
      {
        "_id": 24,
        "follower": 2,
        "following": 4
      },
      {
        "_id": 13,
        "follower": 1,
        "following": 3
      },
      {
        "_id": 23,
        "follower": 2,
        "following": 3
      },
      {
        "_id": 12,
        "follower": 1,
        "following": 2
      }
    ]
  }

我想我需要做一些放松，但我现在有点卡住了

Answer 1

以下是两种可能的方法。(我还没有用更大的数据集进行测试，所以您的里程可能会有所不同！)

第一个构建在您的$graphLookup阶段上：

db.users.aggregate([
  { $match: { _id: 1 }},
  { $graphLookup: {
    from: 'follows',
    startWith: '$_id',
    connectFromField: 'following',
    connectToField: 'follower',
    maxDepth: 1,
    as: 'connections'
  }},
  { $unwind: { path: '$connections' }},
  { $group: {
    _id: '$connections.follower',
    follows: {
      $addToSet: '$connections.following'
    }
  }},
  { $unwind: { path: '$follows' }},
  { $group: {
    _id: '$follows',
    isFollowedBy: {
      $addToSet: '$_id'
    }
  }},
  { $match: { isFollowedBy: { $not: { $in: [1] }} }},
  { $group: {
    _id: null,
    newConnections: {
      $addToSet: '$_id'
    }
  }},
  { $project: { _id: 0 }}
])

请注意，这个管道最终从另一个集合中间构建关系，因此另一个方法是从另一个集合开始，如下所示：

db.follows.aggregate([
  { $lookup: {
    from: 'follows',
    localField: 'following',
    foreignField: 'follower',
    as: 'potentialSet'
  }},
  { $unwind: {
    path: "$potentialSet",
    preserveNullAndEmptyArrays: true
  }},
  { $group: {
    _id: "$follower",
    "alreadyFollowing": {
      $addToSet: "$following"
    },
    "potentialConnections": {
      "$addToSet": "$potentialSet.following"
    }
  }},
  { $project: {
    newConnections: { $setDifference: [ "$potentialConnections", "$alreadyFollowing" ] }
  }},
  { $match: { _id: 1 }},
  { $project: { _id: 0 }}
])

如果有帮助的话，我使用MongoDB指南针社区版来帮助构建这些管道。它非常酷，因为它允许您快速迭代并查看每个阶段的输出，这在调试管道时确实有帮助。