$replaceRoot (집계 단계)

정의

$replaceRoot

Replaces the input document with the specified document. The operation replaces all existing fields in the input document, including the _id field. You can promote an existing embedded document to the top level, or create a new document for promotion (see example).

참고

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> } }

대체 문서는 문서로 해석되는 모든 유효한 표현식 일 수 있습니다. <replacementDocument> 이(가) 문서가 아닌 경우 스테이지 오류가 발생하고 실패합니다. 표현식에 대한 자세한 내용은 표현식을 참조하세요 .

행동

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" } }
])

이 오류를 방지하려면 $mergeObjects를 사용하여 name 문서를 일부 기본 문서와 병합하면 됩니다.

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' }
]

또는 $ifNull 표현식을 사용하여 다른 문서를 루트로 지정할 수 있습니다. 예시:

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 }
]

예시

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 }
])

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 }
]

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 }
      ]
   }
])

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 }
]

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" }
])

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

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

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" }
] )

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'
  }
]

[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; }
}

[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; }
}

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 }

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

자세히 알아보기

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