MongoDB按字段分组并将子文档合并至父文档的实现方法

Got it, let's walk through how to solve this aggregation task in MongoDB. You want to group your documents by the a field, then merge each group's documents into a parent structure using one of the child document's fields as a key. Let's start with clarifying the input and possible desired outputs, then jump into the aggregation pipeline.

原始输入文档 #

First, here's your raw data formatted properly:

[
  { "_id" : "1", "a" : "11", "b" : "o11", "c" : "s11" },
  { "_id" : "2", "a" : "22", "b" : "o22", "c" : "s22" },
  { "_id" : "3", "a" : "11", "b" : "o12", "c" : "s12" },
  { "_id" : "4", "a" : "11", "b" : "o13", "c" : "s13" },
  { "_id" : "5", "a" : "22", "b" : "o23", "c" : "s23" }
]

常见的期望输出结构 #

I'll cover two common scenarios based on what you might mean by "merge subdocuments using a field as the key":

场景1：以b为键，c为对应值

If you want to map each b value directly to its c value under the grouped a field, the result would look like this:

[
  {
    "a": "11",
    "items": {
      "o11": "s11",
      "o12": "s12",
      "o13": "s13"
    }
  },
  {
    "a": "22",
    "items": {
      "o22": "s22",
      "o23": "s23"
    }
  }
]

场景2：以b为键，对应整个子文档

If you want to preserve the full subdocument under each b key, the result would be:

[
  {
    "a": "11",
    "items": {
      "o11": { "_id": "1", "b": "o11", "c": "s11" },
      "o12": { "_id": "3", "b": "o12", "c": "s12" },
      "o13": { "_id": "4", "b": "o13", "c": "s13" }
    }
  },
  {
    "a": "22",
    "items": {
      "o22": { "_id": "2", "b": "o22", "c": "s22" },
      "o23": { "_id": "5", "b": "o23", "c": "s23" }
    }
  }
]

对应的聚合管道 #

Let's build the pipeline for each scenario.

场景1的聚合代码

db.yourCollectionName.aggregate([
  // Step 1: Group by 'a' and collect { key: b, value: c } pairs
  {
    $group: {
      _id: "$a",
      keyValuePairs: {
        $push: {
          k: "$b", // Use 'b' as the key
          v: "$c" // Use 'c' as the corresponding value
        }
      }
    }
  },
  // Step 2: Convert the array of key-value pairs into an object
  {
    $project: {
      _id: 0, // Hide the default _id from grouping
      a: "$_id", // Rename the grouped _id back to 'a'
      items: { $arrayToObject: "$keyValuePairs" } // Turn the array into a nested object
    }
  }
])

场景2的聚合代码

db.yourCollectionName.aggregate([
  // Step 1: Group by 'a' and collect { key: b, value: full document } pairs
  {
    $group: {
      _id: "$a",
      keyDocPairs: {
        $push: {
          k: "$b", // Use 'b' as the key
          v: { // Include all fields from the original document except 'a' (or include it if needed)
            _id: "$_id",
            b: "$b",
            c: "$c"
          }
        }
      }
    }
  },
  // Step 2: Convert the array of key-document pairs into an object
  {
    $project: {
      _id: 0,
      a: "$_id",
      items: { $arrayToObject: "$keyDocPairs" }
    }
  }
])

关键知识点解释 #

• $group: This stage groups documents by the a field, and uses $push to collect the key-value (or key-document) pairs into an array for each group.
• $arrayToObject: This MongoDB operator takes an array of {k: "...", v: "..."} objects and converts it into a single object where each k becomes a key, mapped to its corresponding v.

Just replace yourCollectionName with the actual name of your collection, and adjust the k and v fields in the $push stage if you want to use a different field as the key (e.g., use c as the key instead of b).

内容的提问来源于stack exchange，提问作者vanba06