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$serializeEJSON（表达式操作符）

定义

$serializeEJSON: 8.3版本新增。
将BSON值转换为扩展JSON (EJSON ) 格式。结果是带有EJSON类型包装器的BSON文档，然后可以使用将其转换为JSON字符串。$toString

语法

{
   $serializeEJSON: {
      input: <expression>,
      relaxed: <boolean>,
      onError: <expression>
   }
}

$serializeEJSON 接受包含以下字段的文档：

字段	类型	必要性	说明
`input`	表达式(expression)	必需	要转换为扩展JSON格式的BSON值。
`relaxed`	布尔	Optional	指定是否使用宽松扩展JSON格式。如果为 `true`，`$serializeEJSON` 使用宽松扩展JSON格式，该格式将数字类型（Int32、Int64、Double）表示为原生JSON数字，以提高可读性。如果为 `false` 或未指定，则 `$serializeEJSON` 使用规范扩展JSON格式，该格式保留所有BSON 类型的类型信息。默认： `false`
`onError`	表达式(expression)	Optional	如果操作在转换期间遇到错误，则返回该值。如果未指定，则在发生错误时，操作会引发错误并停止。

行为

规范格式和宽松格式的比较

下表显示了常见BSON 类型如何转换为扩展JSON：

BSON 类型	规范扩展 JSON	宽松扩展 JSON
Int32	`{"$numberInt": "42"}`	`42`
Int64	`{"$numberLong": "42"}`	`42`
double	`{"$numberDouble": "42.5"}`	`42.5`
ObjectId	`{"$oid": "507f1f77bcf86cd799439011"}`	与 Canonical 相同
Date	`{"$date": {"$numberLong": "1609459200000"}}`	`{"$date": "2021-01-01T00:00:00.000Z"}`
二进制文件	`{"$binary": {"base64": "AQIDBA==", "subType": "00"}}`	与 Canonical 相同
正则表达式	`{"$regularExpression": {"pattern": "abc", "options": "i"}}`	与 Canonical 相同

Null 和缺失值

如果 input 值为 null 或缺失，则 $serializeEJSON 返回 null。

示例

本页上的示例使用 sample_mflix示例数据集的数据。有关如何将此数据集加载到自管理MongoDB 部署中的详细信息，请参阅加载示例数据集。如果对示例数据库进行了任何修改，则可能需要删除并重新创建数据库才能运行本页上的示例。

规范扩展JSON示例

以下示例将电影文档转换为规范扩展JSON格式：

db.movies.aggregate([
  {
    $match: { title: "Inception" }
  },
  {
    $project: {
      ejson: { $serializeEJSON: { input: "$$ROOT" } }
    }
  }
])

{
  _id: ObjectId("573a1398f29313caabcea974"),
  ejson: {
    _id: { $oid: "573a1398f29313caabcea974" },
    title: "Inception",
    year: { $numberInt: "2010" },
    runtime: { $numberInt: "148" },
    released: { $date: { $numberLong: "1279238400000" } },
    cast: [
      "Leonardo DiCaprio",
      "Joseph Gordon-Levitt",
      "Ellen Page",
      "Tom Hardy"
    ],
    genres: [ "Action", "Sci-Fi", "Thriller" ],
    directors: [ "Christopher Nolan" ]
  }
}

宽松扩展JSON示例

以下示例使用 relaxed 选项来提高输出的可读性：

db.movies.aggregate([
  {
    $match: { title: "Inception" }
  },
  {
    $project: {
      ejson: {
        $serializeEJSON: {
          input: "$$ROOT",
          relaxed: true
        }
      }
    }
  }
])

{
  _id: ObjectId("573a1398f29313caabcea974"),
  ejson: {
    _id: { $oid: "573a1398f29313caabcea974" },
    title: "Inception",
    year: 2010,
    runtime: 148,
    released: { $date: "2010-07-16T00:00:00.000Z" },
    cast: [
      "Leonardo DiCaprio",
      "Joseph Gordon-Levitt",
      "Ellen Page",
      "Tom Hardy"
    ],
    genres: [ "Action", "Sci-Fi", "Thriller" ],
    directors: [ "Christopher Nolan" ]
  }
}

转换为JSON字符串

要将EJSON结果转换为JSON字符串，请将与$serializeEJSON $toString结合使用：

db.movies.aggregate([
  {
    $match: { title: "The Godfather" }
  },
  {
    $project: {
      title: 1,
      jsonString: {
        $toString: {
          $serializeEJSON: { input: "$$ROOT" }
        }
      }
    }
  }
])

[
  {
    _id: '573a13c5f29313caabd6ee62',
    title: 'The Godfather',
    jsonString: `{"_id":"573a13c5f29313caabd6ee62",` +
      `"fullplot":"When the aging head of a famous crime family decides to transfer his position to one of his subalterns, a series of unfortunate events start happening to the family, and a war begins between all the well-known families leading to insolence, deportation, murder and revenge, and ends with the favorable successor being finally chosen.",` +
      `"imdb":{"rating":9.2,"votes":1038358,"id":68646},` +
      `"year":1972,` +
      `"plot":"The aging patriarch of an organized crime dynasty transfers control of his clandestine empire to his reluctant son.",` +
      `"genres":["Crime","Drama"],` +
      `"rated":"R",` +
      `"metacritic":100,` +
      `"title":"The Godfather",` +
      `"lastupdated":"2015-09-02 00:08:23.680000000",` +
      `"languages":["English","Italian","Latin"],` +
      `"writers":["Mario Puzo (screenplay)","Francis Ford Coppola (screenplay)","Mario Puzo (novel)"],` +
      `"type":"movie",` +
      `"tomatoes":{"website":"http://www.thegodfather.com","viewer":{"rating":4.4,"numReviews":725773,"meter":98},"dvd":{"$date":"2001-10-09T00:00:00.000Z"},"critic":{"rating":9.2,"numReviews":84,"meter":99},"lastUpdated":{"$date":"2015-09-12T17:15:13.000Z"},"consensus":"One of Hollywood's greatest critical and commercial successes, The Godfather gets everything right; not only did the movie transcend expectations, it established new benchmarks for American cinema.","rotten":1,"production":"Paramount Pictures","fresh":83},` +
      `"poster":"https://m.media-amazon.com/images/M/MV5BM2MyNjYxNmUtYTAwNi00MTYxLWJmNWYtYzZlODY3ZTk3OTFlXkEyXkFqcGdeQXVyNzkwMjQ5NzM@._V1_SY1000_SX677_AL_.jpg",` +
      `"num_mflix_comments":131,` +
      `"released":{"$date":"1972-03-24T00:00:00.000Z"},` +
      `"awards":{"wins":33,"nominations":19,"text":"Won 3 Oscars. Another 30 wins & 19 nominations."},` +
      `"countries":["USA"],` +
      `"cast":["Marlon Brando","Al Pacino","James Caan","Richard S. Castellano"],` +
      `"directors":["Francis Ford Coppola"],` +
      `"runtime":175}`
  }
]

注意

为便于阅读，上一示例中的 jsonString字段已重新格式化。在实际输出中， JSON字符串是没有空格的单行。

序列化特定字段

您可以序列化特定字段而不是整个文档：

db.movies.aggregate([
  {
    $match: { year: { $gte: 2010 } }
  },
  {
    $project: {
      title: 1,
      metadataEJSON: {
        $serializeEJSON: {
          input: {
            releaseDate: "$released",
            runtime: "$runtime",
            imdbRating: "$imdb.rating"
          }
        }
      }
    }
  }
])

[
  {
    _id: '573a13c5f29313caabd6ee61',
    title: 'Inception',
    metadataEJSON: {
      releaseDate: { '$date': '2010-07-16T00:00:00.000Z' },
      runtime: 148,
      imdbRating: 8.8
    }
  }
]

使用 onError 进行错误处理

以下示例使用 onError 在序列化失败时提供回退值：

db.movies.aggregate([
  {
    $project: {
      title: 1,
      ejson: {
        $serializeEJSON: {
          input: "$customField",
          onError: { error: "Serialization failed" }
        }
      }
    }
  }
])

了解详情

后退

$second

来年

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