$getField (aggregation)

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Definition定义

$getField

New in version 5.0.在版本5.0中新增。

Returns the value of a specified field from a document. 返回文档中指定字段的值。If you don't specify an object, $getField returns the value of the field from $$CURRENT.如果未指定对象，$getField将从$$CURRENT返回字段的值。

You can use $getField to retrieve the value of fields with names that contain periods (.) or start with dollar signs ($).可以使用$getField检索名称中包含句点（.）或以美元符号（$）开头的字段的值。

Tip提示

Use $setField to add or update fields with names that contain dollar signs ($) or periods (.).使用$setField添加或更新名称包含美元符号（$）或句点（.）的字段。

Syntax语法

$getField has the following syntax:语法如下：

{
  $getField: {
    field: <String>,
    input: <Object>
  }
}
Field字段Type类型Description描述
fieldString

Field in the input object for which you want to return a value. 要为其返回值的input对象中的字段。field can be any valid expression that resolves to a string constant.可以是解析为字符串常量的任何有效表达式。

If field begins with a dollar sign ($), place the field name inside of a $literal expression to return its value.如果field以美元符号（$）开头，请将字段名放在$literal表达式中以返回其值。

inputObject

Default: $$CURRENT

A valid expression that contains the field for which you want to return a value. 包含要为其返回值的field的有效表达式。input must resolve to an object, missing, null, or undefined. 输入必须解析为对象、missing、null或undefined。If omitted, defaults to the document currently being processed in the pipeline ($$CURRENT).如果省略，则默认为管道中当前正在处理的文档（$$CURRENT）。

$getField has the following shorthand syntax for retrieving field values from $$CURRENT:具有以下用于从$$CURRENT检索字段值的简写语法：

{
  $getField: <String>
}

For this syntax, the argument is equivalent to the value of field described above.对于此语法，参数等效于上述field的值。

Behavior行为

  • If field resolves to anything other than a string constant, $getField returns an error.若field解析为字符串常量以外的任何值，$getField将返回错误。
  • If the field that you specify is not present in the input object, or in $$CURRENT if you don't specify an input object, $getField returns missing.如果input对象中不存在指定的field，或者如果未指定input对象，且在$$CURRENT中不存在指定的field，则$getField返回missing。
  • If input evaluates to missing, undefined, or null, $getField returns null.如果input的计算结果为missing、undefined或null，则$getField将返回null。
  • If input evaluates to anything other than an object, missing, undefined, or null, $getField returns an error.如果input的计算结果不是对象、missing、undefined或为null，$getField将返回错误。
  • $getField doesn't implicitly traverse objects or arrays. 不隐式遍历对象或数组。For example, $getField evaluates a field value of a.b.c as a top-level field a.b.c instead of a nested field { a: { b: { c: } } }.例如，$getField将a.b.c字段值计算为顶级字段a.b.c，而不是嵌套字段{ a: { b: { c: } } }。
Tip提示
See also: 参阅：

Considerations for field names字段名注意事项

Examples示例

Query Fields that Contain Periods (.)包含句点（.）的查询字段

Consider an inventory collection with the following documents:考虑使用以下文档进行inventory集合：

{ "_id" : 1, "item" : "sweatshirt", "price.usd": 45.99, qty: 300 }
{ "_id" : 2, "item" : "winter coat", "price.usd": 499.99, qty: 200 }
{ "_id" : 3, "item" : "sun dress", "price.usd": 199.99, qty: 250 }
{ "_id" : 4, "item" : "leather boots", "price.usd": 249.99, qty: 300 }
{ "_id" : 5, "item" : "bow tie", "price.usd": 9.99, qty: 180 }

The following operation uses the $getField and $gt operators to find which products have a price.usd greater than 200:以下操作使用$getField和$gt运算符查找price.usd高于200的产品：

db.inventory.aggregate( [
  {
    $match:
      { $expr:
        { $gt: [ { $getField: "price.usd" }, 200 ] }
      }
   }
] )

The operation returns the following results:该操作返回以下结果：

[
  { _id: 2, item: 'winter coat', qty: 200, 'price.usd': 499.99 },
  { _id: 4, item: 'leather boots', qty: 300, 'price.usd': 249.99 }
]

Query Fields that Start with a Dollar Sign ($)以美元符号（$）开头的查询字段

Consider an inventory collection with the following documents:考虑使用以下文档进行inventory集合：

{ "_id" : 1, "item" : "sweatshirt", "$price": 45.99, qty: 300 }
{ "_id" : 2, "item" : "winter coat", "$price": 499.99, qty: 200 }
{ "_id" : 3, "item" : "sun dress", "$price": 199.99, qty: 250 }
{ "_id" : 4, "item" : "leather boots", "$price": 249.99, qty: 300 }
{ "_id" : 5, "item" : "bow tie", "$price": 9.99, qty: 180 }

The following operation uses the $getField, $gt, and $literal operators to find which products have a $price greater than 200:以下操作使用$getField、$gt和$literal运算符查找$price大于200的产品：

db.inventory.aggregate( [
  {
    $match:
      { $expr:
        { $gt: [ { $getField: {$literal: "$price" } }, 200 ] }
      }
   }
] )

The operation returns the following results:该操作返回以下结果：

[
  { _id: 2, item: 'winter coat', qty: 200, '$price': 499.99 },
  { _id: 4, item: 'leather boots', qty: 300, '$price': 249.99 }
]

Query a Field in a Sub-document查询子文档中的字段

Create an inventory collection with the following documents:使用以下文档创建inventory集合：

db.inventory.insertMany( [
   { "_id" : 1, "item" : "sweatshirt",  "price.usd": 45.99,
     "quantity": { "$large": 50, "$medium": 50, "$small": 25 }
   },
   { "_id" : 2, "item" : "winter coat", "price.usd": 499.99,
     "quantity": { "$large": 35, "$medium": 35, "$small": 35 }
   },
   { "_id" : 3, "item" : "sun dress", "price.usd": 199.99,
     "quantity": { "$large": 45, "$medium": 40, "$small": 5 }
   },
   { "_id" : 4, "item" : "leather boots", "price.usd": 249.99,
     "quantity": { "$large": 20, "$medium": 30, "$small": 40 }
   },
   { "_id" : 5, "item" : "bow tie", "price.usd": 9.99,
     "quantity": { "$large": 0, "$medium": 10, "$small": 75 }
   }
] )

The following operation returns documents where the number of $small items is less than or equal to 20.以下操作返回$small项目数小于或等于20的文档。

db.inventory.aggregate( [
   { $match:
      { $expr:
         { $lte:
            [
               { $getField:
                  { field: { $literal: "$small" },
                    input: "$quantity"
                  }
               },
               20
            ]
         }
      }
   }
] )

Use these operators to query the collection:使用以下运算符查询集合：

  • The $lte operator finds values less than or equal to 20.$lte运算符查找小于或等于20的值。
  • $getField requires explicit field and input parameters because the $small field is part of a sub-document.需要显式field和input参数，因为$small字段是子文档的一部分。
  • $getField uses $literal to evaluate "$small", because the field name has a dollar sign ($) in it.使用$literal计算“$small”，因为字段名中有一个美元符号（$）。

Example output:示例输出：

[
  {
    _id: 3,
    item: 'sun dress',
    'price.usd': 199.99,
    quantity: { '$large': 45, '$medium': 40, '$small': 5 }
  }
]
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