MongoDB $arrayElemAt 返回错误的值

Question

我的聚合查询中有这个简单的$project 用于返回每个 产品项的第一个图像（字符串）元素并将其存储在first 字段中。

{
    $project: {
      'products._id': 1,
      'products.images': 1,
      'products.first': { $arrayElemAt: ['$products.images', 0] },
    },
  }

但是，当我执行查询时，它会将 first 字段设置为第一个产品的整个图像数组的值。

期望的结果：

{
  "_id": "60d9adda7f017440403d57fb",
  "products": [
    {
      "_id": "5e48a95545f3350017aecce0",
      "images": ["image1","image2"],
      "first": "image1"
    },
    {
      "_id": "5e4c0b0986c1d0001757ae06",
      "images": ["image3"],
      "first": "image3"
    },
    {
      "_id": "5e4c0e1c86c1d0001757ae07",
      "images": ["image4"],
      "first": "image4"
    }
  ]
}

实际结果：

{
  "_id": "60d9adda7f017440403d57fb",
  "products": [
    {
      "_id": "5e48a95545f3350017aecce0",
      "images": ["image1","image2"],
      "first": ["image1", "image2"]
    },
    {
      "_id": "5e4c0b0986c1d0001757ae06",
      "images": ["image3"],
      "first": ["image1", "image2"]
    },
    {
      "_id": "5e4c0e1c86c1d0001757ae07",
      "images": ["image4"],
      "first": ["image1", "image2"]
    }
  ]
}

请问，我该如何解决？

谢谢。

Answer 1

有一些关于它的讨论here 和here。这是字段路径表达式的问题，因此您不能那样投影。要获得预期的输出，您可以使用$map:

db.collection.aggregate([
  {
    $project: {
      products: {
        $map: {
          input: "$products",
          as: "product",
          in: {
            _id: "$$product._id",
            images: "$$product.images",
            first: {
              "$arrayElemAt": [ "$$product.images", 0 ]
            }
          }
        }
      }
    },
  }
])

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