从数组 mongodb 中的特定对象元素投影一个字段

Question

我有一个集合：

{
    "_id": 1,
    "_deleted": false,
    "customFields": [{
        "fieldName": "sapID",
        "value": ""
    }, {
        "fieldName": "salesTerritory",
        "value": ""
    }, {
        "fieldName": "clientType",
        "value": "Corporate"
    }],
}

如何project(aggregate)仅使用fieldName =“clientType”元素的value字段：

db.collection.aggregate(
    {
        $project:{value:<code>}
    }
)

我尝试了< code>$filter 但它不起作用

Answer 1

db.collection.aggregate([
  {
    $project: {
      "customFields.value": 1
    }
  }
])

mongoplayground

---

db.collection.aggregate([
  {
    $set: {
      customFields: {
        $map: {
          input: "$customFields",
          as: "c",
          in: {
            $cond: {
              if: { "$eq": [ "$c.fieldName", "clientType" ] },
              then: { value: "$c.value" },
              else: "$c"
            }
          }
        }
      }
    }
  }
])

mongoplayground

Answer 2

这又如何呢？

db.collection.aggregate([
   {
      $project: {
         customFields: {
            $filter: {
               input: "$customFields",
               $cond: { $eq: ["$this.fieldName", "clientType"] }
            }
         }
      }
   }
])

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