MongoDB - 复杂嵌套数组中的 $lookup

Question

我正在 mongodb 中构建一个新的 LMS，我有以下集合：

课程

{
  "_id": ObjectId("5f6a6b159de1304fb885b194"),
  "title": "Course test",
  "sections": [
    {
      "_id": ObjectId("5f6a6b159de1304fb885b195"),
      "title": "Section 1 - introduction",
      "order": 1,
      "modules": [
        {
          "_id": ObjectId("5f6a6b159de1304fb885b196"),
          "module_FK_id": ObjectId("5f6a6b149de1304fb885b135"),
          "title": "Module 1",
          "order": 1
        },
        {
          "_id": ObjectId("5f6a6b159de1304fb885b198"),
          "module_FK_id": ObjectId("5f6a6b149de1304fb885b14a"),
          "title": "Module 2",
          "order": 2
        },      
      ]
    },
    {
      "_id": ObjectId("5f6a6b149de1304fb885b175"),
      "title": "Section 2 - How to do something",
      "order": 2,
      "modules": [
        {
          "_id": ObjectId("5f6a6b149de1304fb885b141"),
          "module_FK_id": ObjectId("5f6a6b149de1304fb885b150"),
          "title": "Module 1",
          "order": 1
        },
        {
          "_id": ObjectId("5f6a6b149de1304fb885b15f"),
          "module_FK_id": ObjectId("5f6a6b149de1304fb885b18e"),
          "title": "Module 2",
          "order": 2
        },      
      ]
    },
  ]
}

模块（仅作为示例）

{
  "_id": ObjectId("5f6a6b149de1304fb885b135"),
  "text": "Lorem ipsum...",
  "mediaUrl": "urllinkhere"
}

如图所示，我选择为章节和模块标题嵌入文档，但我还需要第二个集合，模块，因为每个模块都包含大量文本，我的课程文档可能会很快变得太大。

现在我需要重建整个文档，就好像它完全嵌入一样。 这是一个例子：

   {
      "_id": ObjectId("5f6a6b159de1304fb885b194"),
      "title": "Course test",
      "sections": [
        {
          "_id": ObjectId("5f6a6b159de1304fb885b195"),
          "title": "Section 1 - introduction",
          "order": 1,
          "modules": [
            {
              "_id": ObjectId("5f6a6b159de1304fb885b196"),
              "module_FK_id": ObjectId("5f6a6b149de1304fb885b135"),
              "title": "Module 1",
              "order": 1
              "text": "Lorem ipsum...",
              "mediaUrl": "urllinkhere"
            },
            // last two fields from collection "modules"

我正在尝试聚合和查找的不同组合，但我无法获得所需的结果。

谁能帮帮我？

Answer 1

您可以像下面这样构造聚合管道，just remember to $group in the reverse order of $unwind operations

db.courses.aggregate([
  {
    $unwind: "$sections"
  },
  {
    $unwind: "$sections.modules"
  },
  {
    "$lookup": {
      "from": "modules",
      "localField": "sections.modules.module_FK_id",
      "foreignField": "_id",
      "as": "module_details"
    }
  },
  {
    $unwind: {
      path: "$module_details",
      preserveNullAndEmptyArrays: true
    }
  },
  {
    "$project": {
      title: 1,
      sections: {
        _id: "$sections._id",
        modules: {
          _id: "$sections.modules._id",
          module_FK_id: "$sections.modules.module_FK_id",
          order: "$sections.modules.order",
          title: "$sections.modules.title",
          mediaUrl: "$module_details.mediaUrl",
          text: "$module_details.text"
        },
        order: "$sections.order",
        title: "$sections.title"
      }
    }
  },
  {
    "$group": {
      "_id": "$sections._id",
      main_id: {
        $first: "$_id"
      },
      main_title: {
        $first: "$title"
      },
      order: {
        $first: "$sections.order"
      },
      title: {
        $first: "$sections.title"
      },
      modules: {
        $push: "$sections.modules"
      }
    }
  },
  {
    "$project": {
      "_id": "$main_id",
      "title": "$main_title",
      section: {
        _id: "$_id",
        modules: "$modules",
        title: "$title",
        order: "$order",
        
      }
    }
  },
  {
    $group: {
      _id: "$_id",
      title: {
        $first: "$title"
      },
      sections: {
        $push: "$section"
      }
    }
  }
])

Working Example