Mongodb - 如何在第二个文档中按键连接两个文档并将子文档作为数组合并到父文档

Question

I have three documents written in mongodb. It is mostly like a folder and file structure. I'm trying to search a category of files by inputting a category Id.

And if any files matches with the category id I need to list these files inside the parent folder document as an embedded array in response.

Document folders

[
    {
        _id: "5f7763103a4af84960f86ae6",
        folderName: "abcd"
    },
    {
        _id: "5f7763103a4af84960f86ae7",
        folderName: "qwerty"
    },
    {
        _id: "5f7763103a4af84960f86ae8",
        folderName: "asdfgh"
    },
]

Document files

[
    {
        _id: "1c7763103a4af84960f86aa5",
        fileName: "test file 1",
        folderId: "5f7763103a4af84960f86ae7",
        categoryId: "6b7763103a4af84960f86ae2"
    },
    {
        _id: "1c7763103a4af84960f86aa6",
        fileName: "test file 2",
        folderId: "5f7763103a4af84960f86ae7",
        categoryId: "6b7763103a4af84960f86ae2"
    },
    {
        _id: "1c7763103a4af84960f86aa7",
        fileName: "test file 3",
        folderId: "5f7763103a4af84960f86ae6",
        categoryId: "6b7763103a4af84960f86ae2"
    },
]

Document categories

[
    {
        _id: "6b7763103a4af84960f86ae1",
        categoryName: "misc"
    },
    {
        _id: "6b7763103a4af84960f86ae2",
        categoryName: "images"
    },
    {
        _id: "6b7763103a4af84960f86ae3",
        categoryName: "other"
    },
]

When I search by a categoryId in files document I need to group all the matching files into an array grouped with the parent folder name.

I'm expecting a result like below

    {
        folder: {
            _id: "5f7763103a4af84960f86ae7",
            folderName: "qwerty",
            files: [
                {
                    _id: "1c7763103a4af84960f86aa5",
                    fileName: "test file 1",
                    folderId: "5f7763103a4af84960f86ae7",
                    categoryId: "6b7763103a4af84960f86ae2"
                },
                {
                      _id: "1c7763103a4af84960f86aa6",
                      fileName: "test file 2",
                      folderId: "5f7763103a4af84960f86ae7",
                      categoryId: "6b7763103a4af84960f86ae2"
                  },
                  {
                      _id: "1c7763103a4af84960f86aa7",
                      fileName: "test file 3",
                      folderId: "5f7763103a4af84960f86ae6",
                      categoryId: "6b7763103a4af84960f86ae2"
                  },
            ]
        }
    }

Answer 1

You can do it like this:

db.folders.aggregate([
  {
    "$lookup": {
      "from": "files",
      "localField": "_id",
      "foreignField": "folderId",
      "as": "files"
    }
  }
])

Here is the working example: https://mongoplayground.net/p/HaYRsK0ulXN