MongoDB按嵌套数组排序

Question

我现在使用mongodb来获得我想要的东西。

{
  "_id" : ObjectId("533b66f3e4b038c01f6a427f"),
  "ccNameCount" : [{
      "ccName" : "ATCC",
      "paperCount" : 4660,
      "strainCount" : 9339,
      "version" : 1
    }, {
      "ccName" : "BCC",
      "paperCount" : 24,
      "strainCount" : 41
    }, {
      "ccName" : "BCCM/DCG",
      "paperCount" : 3,
      "strainCount" : 7
    }]
}

示例文档就是这样。现在，我想查询ccNameCount.ccName=ATCC的文档，并按paperCount对结果进行排序。但当然，由于ccNameCount是一个数组，所以只按ccNameCount键排序不会得到我想要的结果。所以，我的问题是，我能做什么？使用MapReduce？

Answer 1

您的最佳选择是聚合框架：

db.collection.aggregate([
    // Match the "documents" that contain your condition
    // reduces the working size
    { "$match": {
        "ccNameCount.ccName": "ATCC"
    }},
    // Keep the original document, you might not really care about this
    { "$project": {
        "_id": {
            "_id": "$_id",
            "ccNameCount": "$ccNameCount"
        },
        "ccNameCount": 1
   }},
    // Unwind the array to de-normalize
    { "$unwind": "$ccNameCount" },
    // Now just match those elements (now documents) as your condition
    { "$match": {
        "ccNameCount.ccName": "ATCC"
    }},
    // Sort the results on the now selected "paperCount"
    { "$sort": { "ccNameCount.paperCount": -1 } },
    // Put the documents back to normal if you want
    { "$project": {
        "_id": "$_id._id",
        "ccNameCount": "$_id.ccNameCount"
    }}
])

因此，在这实际上"选择"了匹配的数组元素之后，结果就可以按照这个位置的字段进行排序。如果您想在结果中返回数组的原始状态，$project阶段是可选的，否则阶段只选择匹配的条目并正确排序