$literal

运算符 $literal 返回指定值，而不将其分析为表达式。 如果需要在聚合管道中包含可能解释为字段名称、运算符或表达式的文本值，则此运算符非常有用。 它确保该值被视为常量，而不是被计算。

Syntax

{
  $literal: <value>
}

参数

例子

请考虑商店集合中的这个示例文档。

{
    "_id": "0fcc0bf0-ed18-4ab8-b558-9848e18058f4",
    "name": "First Up Consultants | Beverage Shop - Satterfieldmouth",
    "location": {
        "lat": -89.2384,
        "lon": -46.4012
    },
    "staff": {
        "totalStaff": {
            "fullTime": 8,
            "partTime": 20
        }
    },
    "sales": {
        "totalSales": 75670,
        "salesByCategory": [
            {
                "categoryName": "Wine Accessories",
                "totalSales": 34440
            },
            {
                "categoryName": "Bitters",
                "totalSales": 39496
            },
            {
                "categoryName": "Rum",
                "totalSales": 1734
            }
        ]
    },
    "promotionEvents": [
        {
            "eventName": "Unbeatable Bargain Bash",
            "promotionalDates": {
                "startDate": {
                    "Year": 2024,
                    "Month": 6,
                    "Day": 23
                },
                "endDate": {
                    "Year": 2024,
                    "Month": 7,
                    "Day": 2
                }
            },
            "discounts": [
                {
                    "categoryName": "Whiskey",
                    "discountPercentage": 7
                },
                {
                    "categoryName": "Bitters",
                    "discountPercentage": 15
                },
                {
                    "categoryName": "Brandy",
                    "discountPercentage": 8
                },
                {
                    "categoryName": "Sports Drinks",
                    "discountPercentage": 22
                },
                {
                    "categoryName": "Vodka",
                    "discountPercentage": 19
                }
            ]
        },
        {
            "eventName": "Steal of a Deal Days",
            "promotionalDates": {
                "startDate": {
                    "Year": 2024,
                    "Month": 9,
                    "Day": 21
                },
                "endDate": {
                    "Year": 2024,
                    "Month": 9,
                    "Day": 29
                }
            },
            "discounts": [
                {
                    "categoryName": "Organic Wine",
                    "discountPercentage": 19
                },
                {
                    "categoryName": "White Wine",
                    "discountPercentage": 20
                },
                {
                    "categoryName": "Sparkling Wine",
                    "discountPercentage": 19
                },
                {
                    "categoryName": "Whiskey",
                    "discountPercentage": 17
                },
                {
                    "categoryName": "Vodka",
                    "discountPercentage": 23
                }
            ]
        }
    ]
}

示例 1：添加文本状态字段

此查询演示如何添加文本状态信息来存储文档，包括可能解释为字段名称或运算符的文本字符串。

db.stores.aggregate([
  { $match: {"_id": "2cf3f885-9962-4b67-a172-aa9039e9ae2f"} },
  {
    $project: {
      name: 1,
      totalSales: "$sales.salesByCategory.totalSales",
      status: { $literal: "Active" },
      reviewStatus: { $literal: "$pending" },
      priority: { $literal: 1 },
      metadata: {
        $literal: {
          lastUpdated: "2024-06-13",
          source: "automated-system"
        }
      }
    }
  }
])

此查询返回以下结果。

[
  {
    "_id": "2cf3f885-9962-4b67-a172-aa9039e9ae2f",
    "name": "First Up Consultants | Bed and Bath Center - South Amir",
    "totalSales": 37701,
    "status": "Active",
    "reviewStatus": "$pending",
    "priority": 1,
    "metadata": {
      "lastUpdated": "2024-06-13",
      "source": "automated-system"
    }
  }
]

示例 2：创建文本数组和条件值

此查询演示如何在 $literal 数组和条件表达式中使用，以确保不会将值解释为运算符。

db.stores.aggregate([
  { $match: {"_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74"} },
  {
    $project: {
      name: 1,
      totalSales: "$sales.salesByCategory.totalSales",
      performanceCategory: {
        $cond: {
          if: { $gte: ["$sales.totalSales", 100000] },
          then: { $literal: "High Performer" },
          else: { $literal: "Standard" }
        }
      },
      tags: { $literal: ["$featured", "$promoted", "$new"] },
      searchKeywords: {
        $literal: {
          "$or": ["electronics", "entertainment"],
          "$and": ["home", "theater"]
        }
      }
    }
  }
])

此查询返回以下结果。

[
  {
    "_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74",
    "name": "Proseware, Inc. | Home Entertainment Hub - East Linwoodbury",
    "totalSales": 160000,
    "performanceCategory": "High Performer",
    "tags": ["$featured", "$promoted", "$new"],
    "searchKeywords": {
      "$or": ["electronics", "entertainment"],
      "$and": ["home", "theater"]
    }
  }
]

示例 3：文本 null 值和布尔值

此查询演示如何使用 $literal null 值和布尔值，当需要显式设置这些值而不是计算这些值时尤其有用。

db.stores.aggregate([
  { $match: {"_id": "e6895a31-a5cd-4103-8889-3b95a864e5a6"} },
  {
    $project: {
      name: 1,
      totalSales: "$sales.salesByCategory.totalSales",
      specialOffer: { $literal: null },
      isOnline: { $literal: false },
      hasInventory: { $literal: true },
      calculationFormula: { $literal: "$multiply: [price, quantity]" },
      ratingSystem: {
        $literal: {
          min: 0,
          max: 5,
          default: null
        }
      }
    }
  }
])

此查询返回以下结果。

[
  {
    "_id": "e6895a31-a5cd-4103-8889-3b95a864e5a6",
    "name": "VanArsdel, Ltd. | Picture Frame Store - Port Clevelandton",
    "totalSales": 17676,
    "specialOffer": null,
    "isOnline": false,
    "hasInventory": true,
    "calculationFormula": "$multiply: [price, quantity]",
    "ratingSystem": {
      "min": 0,
      "max": 5,
      "default": null
    }
  }
]

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