mongo 和 node 中嵌套数组的投影

发布于 01-14 04:19 字数 415 浏览 4 评论 0原文

大家好，我正在尝试通过采用国家代码和州代码来预测属于特定州的城市

[
  {
    '$match': {
      'iso2': 'IN'
    }
  }, {
    '$project': {
      'states': {
        '$slice': [
          '$states.cities.name', 1, 1
        ]
      }
    }
  }
]

当我尝试这个时，我得到了结果，但是有没有更好的方法来做到这一点 tq 在此处输入图像描述

原文

hi everyone i am trying to project the cities which are belongs to particular state by taking the country_code and state_code

[
  {
    '$match': {
      'iso2': 'IN'
    }
  }, {
    '$project': {
      'states': {
        '$slice': [
          '$states.cities.name', 1, 1
        ]
      }
    }
  }
]

when i tried this i am getting the result but is there any better way to do it
tq enter image description here

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笔落惊风雨2025-01-21 04:19:17

我会按如下方式执行此操作：

db.collection.aggregate([
{
      "$match": {
      "iso2": "IN",
      "states.state_code": "AG"
}
},
{
  $addFields: {
    states: {
     "$filter": {
       "input": "$states",
       "as": "state",
       "cond": {
         "$eq": [
           "$state.state_code",
           "AG"
        ]
       }
      }
     }
    }
   },
   {
     $project: {
       cities: "$states.cities.name"
   }
   },
  {
    $unwind: "$cities"
  }
])

解释：

仅匹配具有 iso2=IN 和 states.state_code=AG 的文档
（对于此阶段来说，如果您至少在 {iso2:1} 上有索引或在 {iso2:1,states.state_code:1} 上有复合索引，则效果很好）
仅过滤 state_code=AG 的州
$project 仅名称
$unwind 来展平第一个数组。

游乐场

I would do it as follow:

db.collection.aggregate([
{
      "$match": {
      "iso2": "IN",
      "states.state_code": "AG"
}
},
{
  $addFields: {
    states: {
     "$filter": {
       "input": "$states",
       "as": "state",
       "cond": {
         "$eq": [
           "$state.state_code",
           "AG"
        ]
       }
      }
     }
    }
   },
   {
     $project: {
       cities: "$states.cities.name"
   }
   },
  {
    $unwind: "$cities"
  }
])

Explained:

Match only documents having iso2=IN and states.state_code=AG
( For this stage is good if you have index on at least {iso2:1} or compound index on {iso2:1,states.state_code:1} )
Filter only the states with state_code=AG
$project the names only
$unwind to flatten first array.

playground

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