使用查找和嵌套对象数组进行聚合查询时数组出错

Question

我有一个无法 100% 解决的查询问题

事实上，当用户在帖子中没有任何评论时。在 cmets 内部有一个“createdBy”，我需要在数组中查找该用户。如果没有 cmets，它返回一个带有空对象的数组，但它必须返回一个空数组，而不是空对象。

谁能帮帮我？非常感谢您！

这里是我的用户集合（数据）

[
  {
    _id: ObjectId("619d0f5df3f74665aff1a551"),
    name: "Test Name",
    surname: "Test Surname2",
    createdAt: ISODate("2021-11-11T17:21:58.624+01:00"),
    updatedAt: ISODate("2021-11-25T10:35:25.842+01:00"),
    posts: [
      {
        _id: ObjectId("619d0f5df3f74575aff1a551"),
        updatedAt: ISODate("2021-11-23T16:57:17.816+01:00"),
        createdAt: ISODate("2021-11-23T16:57:17.816+01:00"),
        content: "Test content....",
        comments: [
          {
            createdBy: ObjectId("618d4326f1668007b3b98404"),
            comment: "test comment...",
            _id: ObjectId("619dfaaaa88266dc91b9489c"),
          },
          {
            createdBy: ObjectId("618d4326f1668007b3b98404"),
            comment: "test comment...",
            _id: ObjectId("619dfc60a88266dc91b95741"),
          },
        ],
        date: ISODate("2021-11-23T16:57:17.820+01:00"),
      },
      {
        _id: ObjectId("619d0f5df3f74575aff1a551"),
        updatedAt: ISODate("2021-11-23T16:57:17.816+01:00"),
        createdAt: ISODate("2021-11-23T16:57:17.816+01:00"),
        content: "Test content....",
        comments: [],
        date: ISODate("2021-11-23T16:57:17.820+01:00"),
      },
    ],
  },
  {
    _id: ObjectId("619d0f5df3f74665aff1a551"),
    name: "Test Name",
    surname: "test surname",
    createdAt: ISODate("2021-11-11T17:21:58.624+01:00"),
    updatedAt: ISODate("2021-11-25T10:35:25.842+01:00"),
    posts: [
      {
        _id: ObjectId("619d0f5df3f74575aff1a551"),
        updatedAt: ISODate("2021-11-23T16:57:17.816+01:00"),
        createdAt: ISODate("2021-11-23T16:57:17.816+01:00"),
        content: "Test content....",
        comments: [
          {
            createdBy: ObjectId("618d4326f1668007b3b98404"),
            comment: "test comment...",
            _id: ObjectId("619dfaaaa88266dc91b9489c"),
          },
          {
            createdBy: ObjectId("618d4326f1668007b3b98404"),
            comment: "test comment...",
            _id: ObjectId("619dfe7ba88266dc91b961b6"),
          },
        ],
        date: ISODate("2021-11-23T16:57:17.820+01:00"),
      },
      {
        _id: ObjectId("619d0f5df3f74575aff1a551"),
        updatedAt: ISODate("2021-11-23T16:57:17.816+01:00"),
        createdAt: ISODate("2021-11-23T16:57:17.816+01:00"),
        content: "Test content....",
        comments: [
          {
            createdBy: ObjectId("618d4326f1668007b3b98404"),
            comment: "test comment...",
            _id: ObjectId("619dfaaaa88266dc91b9489c"),
          },
          {
            createdBy: ObjectId("618d4326f1668007b3b98404"),
            comment: "test comment...",
            _id: ObjectId("619dfc60a88266dc91b95741"),
          },
        ],
        date: ISODate("2021-11-23T16:57:17.820+01:00"),
      },
    ],
  },
];

这里是我的聚合查询

db.users.aggregate([
    { $unwind: { path: '$posts', preserveNullAndEmptyArrays: true } },
    { $unwind: { path: '$posts.comments', preserveNullAndEmptyArrays: true } },
    {
        $lookup: {
            from: 'users',
            localField: 'posts.comments.createdBy',
            foreignField: '_id',
            as: 'posts.comments.createdBy'
        }
    },
    { $unwind: { path: '$posts.comments.createdBy', preserveNullAndEmptyArrays: true } },
    {
        $group: {
            _id: { _id: '$_id', post_id: '$posts._id' },
            name: { $first: '$name' },
            posts: { $push: '$posts' },
            comments: { $push: '$posts.comments' },
        }
    },
    {
        $group: {
            _id: '$_id._id',
            name: { $first: '$name' },
            posts: {
                $push: {
                    _id: '$_id.post_id',
                    date: { $first: '$posts.date' },
                    content: { $first: '$posts.content' },
                    comments: '$comments'
                }
            }
        }
    },
])

这是一个带有失败数组的图像：

Answer 1

您必须从展开中删除 preserveNullAndEmptyArrays 字段才能没有空对象