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$replaceRoot（聚合阶段）

定义

$replaceRoot

将输入文档替换为指定的文档。该操作会替换输入文档中的所有现有字段，包括 _id 字段。您可以将现有的嵌入式文档提升到顶层，或创建新文档进行提升（请参阅示例）。

注意

You can also use the $replaceWith stage. The $replaceWith stage peforms the same action as the $replaceRoot stage, but the stages have different forms.

The $replaceRoot stage has the following form:

{ $replaceRoot: { newRoot: <replacementDocument> } }

替换文档可以是任何解析为文档的有效表达式。如果 <replacementDocument> 不是文档，则该阶段会出错并失败。有关表达式的更多信息，请参阅表达式。

行为

If the <replacementDocument> is not a document, $replaceRoot errors and fails.

If the <replacementDocument> resolves to a missing document (i.e. the document does not exist), $replaceRoot errors and fails. For example, create a collection with the following documents:

db.collection.insertMany([
   { "_id": 1, "name" : { "first" : "John", "last" : "Backus" } },
   { "_id": 2, "name" : { "first" : "John", "last" : "McCarthy" } },
   { "_id": 3, "name": { "first" : "Grace", "last" : "Hopper" } },
   { "_id": 4, "firstname": "Ole-Johan", "lastname" : "Dahl" },
])

Then the following $replaceRoot operation fails because one of the documents does not have the name field:

db.collection.aggregate([
   { $replaceRoot: { newRoot: "$name" } }
])

为了避免该错误，您可以使用$mergeObjects将name文档合并到某个默认文档中；例如：

db.collection.aggregate([
   { $replaceRoot: { newRoot: { $mergeObjects: [ { _id: "$_id", first: "", last: "" }, "$name" ] } } }
])

[
  { _id: 1, first: 'John', last: 'Backus' },
  { _id: 2, first: 'John', last: 'McCarthy' },
  { _id: 3, first: 'Grace', last: 'Hopper' },
  { _id: 4, first: '', last: '' }
]

Alternatively, you can skip the documents that are missing the name field by including a $match stage to check for existence of the document field before passing documents to the $replaceRoot stage:

db.collection.aggregate([
   { $match: { name : { $exists: true, $not: { $type: "array" }, $type: "object" } } },
   { $replaceRoot: { newRoot: "$name" } }
])

[
  { first: 'John', last: 'Backus' },
  { first: 'John', last: 'McCarthy' },
  { first: 'Grace', last: 'Hopper' }
]

也可以使用 $ifNull 表达式指定其他文档作为根文档；例如：

db.collection.aggregate([
   { $replaceRoot: { newRoot: { $ifNull: [ "$name", { _id: "$_id", missingName: true} ] } } }
])

[
  { first: 'John', last: 'Backus' },
  { first: 'John', last: 'McCarthy' },
  { first: 'Grace', last: 'Hopper' },
  { _id: 4, missingName: true }
]

示例

`$replaceRoot` 具有嵌入式文档字段

一个名为 people 的集合包含以下文档：

db.people.insertMany([
   { "_id" : 1, "name" : "Arlene", "age" : 34, "pets" : { "dogs" : 2, "cats" : 1 } },
   { "_id" : 2, "name" : "Sam", "age" : 41, "pets" : { "cats" : 1, "fish" : 3 } },
   { "_id" : 3, "name" : "Maria", "age" : 25 }
])

The following operation uses the $replaceRoot stage to replace each input document with the result of a $mergeObjects operation. The $mergeObjects expression merges the specified default document with the pets document.

db.people.aggregate( [
   { $replaceRoot: { newRoot: { $mergeObjects:  [ { dogs: 0, cats: 0, birds: 0, fish: 0 }, "$pets" ] } } }
] )

[
  { dogs: 2, cats: 1, birds: 0, fish: 0 },
  { dogs: 0, cats: 1, birds: 0, fish: 3 },
  { dogs: 0, cats: 0, birds: 0, fish: 0 }
]

`$replaceRoot` 使用嵌套在数组中的文档

一个名为 students 的集合包含以下文档：

db.students.insertMany([
   {
      "_id" : 1,
      "grades" : [
         { "test": 1, "grade" : 80, "mean" : 75, "std" : 6 },
         { "test": 2, "grade" : 85, "mean" : 90, "std" : 4 },
         { "test": 3, "grade" : 95, "mean" : 85, "std" : 6 }
      ]
   },
   {
      "_id" : 2,
      "grades" : [
         { "test": 1, "grade" : 90, "mean" : 75, "std" : 6 },
         { "test": 2, "grade" : 87, "mean" : 90, "std" : 3 },
         { "test": 3, "grade" : 91, "mean" : 85, "std" : 4 }
      ]
   }
])

以下操作将 grade 字段大于或等于 90 的嵌入式文档提升到顶层：

db.students.aggregate( [
   { $unwind: "$grades" },
   { $match: { "grades.grade" : { $gte: 90 } } },
   { $replaceRoot: { newRoot: "$grades" } }
] )

[
  { test: 3, grade: 95, mean: 85, std: 6 },
  { test: 1, grade: 90, mean: 75, std: 6 },
  { test: 3, grade: 91, mean: 85, std: 4 }
]

`$replaceRoot` （新建文档）

您还可以在 $replaceRoot 阶段创建新文档，并用它们替换所有其他字段。

一个名为 contacts 的集合包含以下文档：

db.contacts.insertMany([
   { "_id" : 1, "first_name" : "Gary", "last_name" : "Sheffield", "city" : "New York" },
   { "_id" : 2, "first_name" : "Nancy", "last_name" : "Walker", "city" : "Anaheim" },
   { "_id" : 3, "first_name" : "Peter", "last_name" : "Sumner", "city" : "Toledo" }
])

以下操作根据 first_name 和 last_name 字段创建新文档。

db.contacts.aggregate( [
   {
      $replaceRoot: {
         newRoot: {
            full_name: {
               $concat : [ "$first_name", " ", "$last_name" ]
            }
         }
      }
   }
] )

[
  { full_name: 'Gary Sheffield' },
  { full_name: 'Nancy Walker' },
  { full_name: 'Peter Sumner' }
]

`$replaceRoot` 使用一份从 `$$ROOT` 创建的新文档和一份默认文档

一个名为 contacts 的集合包含以下文档：

db.contacts.insertMany( [
   { "_id" : 1, name: "Fred", email: "fred@example.net" },
   { "_id" : 2, name: "Frank N. Stine", cell: "012-345-9999" },
   { "_id" : 3, name: "Gren Dell", home: "987-654-3210", email: "beo@example.net" }
] )

The following operation uses $replaceRoot with $mergeObjects to output current documents with default values for missing fields:

db.contacts.aggregate( [
   { $replaceRoot:
      { newRoot:
         { $mergeObjects:
             [
                { _id: "", name: "", email: "", cell: "", home: "" },
                "$$ROOT"
             ]
          }
      }
   }
] )

[
  { _id: 1, name: 'Fred', email: 'fred@example.net', cell: '', home: '' },
  {
    _id: 2,
    name: 'Frank N. Stine',
    email: '',
    cell: '012-345-9999',
    home: ''
  },
  {
    _id: 3,
    name: 'Gren Dell',
    email: 'beo@example.net',
    cell: '',
    home: '987-654-3210'
  }
]

本页上的C#示例使用Atlas示例数据集中的 sample_mflix数据库。要学习；了解如何创建免费的MongoDB Atlas 群集并加载示例数据集，请参阅MongoDB .NET/ C#驱动程序文档中的入门。

以下 Movie 类对 sample_mflix.movies 集合中的文档进行建模：

[BsonIgnoreExtraElements]
public class Movie
{
    [BsonId]
    public ObjectId Id { get; set; }
    [BsonElement("title")]
    public string Title { get; set; } = null!;
    [BsonElement("year")]
    public int? Year { get; set; }
    [BsonElement("runtime")]
    public int? Runtime { get; set; }
    [BsonElement("rated")]
    public string? Rated { get; set; }
    [BsonElement("metacritic")]
    public int Metacritic { get; set; }
    [BsonElement("plot")]
    public string? Plot { get; set; }
    [BsonElement("type")]
    public string? Type { get; set; }
    [BsonElement("cast")]
    public string[]? Cast { get; set; }
    [BsonElement("directors")]
    public string[]? Directors { get; set; }
    [BsonElement("writers")]
    public string[]? Writers { get; set; }
    [BsonElement("imdb")]
    public ImdbData? Imdb { get; set; }
}

以下 ImdbData 类对嵌入的 imdb 文档进行建模：

[BsonIgnoreExtraElements]
public class ImdbData
{
    [BsonElement("id")]
    public int? ImdbId { get; set; }
    [BsonElement("rating")]
    public double? Rating { get; set; }
    [BsonElement("votes")]
    public int? Votes { get; set; }
}

To use the MongoDB .NET/C# driver to add a $replaceRoot stage to an aggregation pipeline, call the UnionWith() method on a PipelineDefinition object.

以下示例创建一个管道阶段，用于过滤器具有 imdb 字段的 Movie 文档，按标题对其进行排序，将结果限制为五个文档，并将每个文档替换为其嵌入的 ImdbData 文档：

var pipeline = new EmptyPipelineDefinition<Movie>()
    .Match(Builders<Movie>.Filter.Exists(m => m.Imdb))
    .Sort(Builders<Movie>.Sort.Ascending(m => m.Title))
    .Limit(5)
    .ReplaceRoot(m => m.Imdb);

{ "rating" : "...", "votes" : 142 }
{ "rating" : "...", "votes" : 77 }
{ "rating" : "...", "votes" : 1608 }
{ "rating" : "...", "votes" : 2532 }
{ "rating" : "...", "votes" : 24645 }

本页上的 Node.js 示例使用 Atlas 示例数据集中的 sample_mflix数据库。要学习如何创建免费的MongoDB Atlas 集群并加载示例数据集，请参阅MongoDB Node.js驱动程序文档中的入门。

要使用MongoDB Node.js驱动程序将 $replaceRoot 阶段添加到聚合管道，请在管道对象中使用 $replaceRoot操作符。

以下示例创建一个管道阶段，该阶段将每个输入 movie 文档替换为存储在其 imdb 属性中的文档。然后，示例运行聚合管道：

const pipeline = [{ $replaceRoot: { newRoot: "$imdb" } }];
const cursor = collection.aggregate(pipeline);
return cursor;

了解详情

To learn more about related pipeline stages, see the $replaceRoot guide.

$replaceRoot（聚合阶段）

定义

行为

示例

$replaceRoot 具有嵌入式文档字段

$replaceRoot 使用嵌套在数组中的文档

$replaceRoot （新建文档）

$replaceRoot 使用一份从 $$ROOT 创建的新文档和一份默认文档

了解详情

`$replaceRoot` 具有嵌入式文档字段

`$replaceRoot` 使用嵌套在数组中的文档

`$replaceRoot` （新建文档）

`$replaceRoot` 使用一份从 `$$ROOT` 创建的新文档和一份默认文档