$replaceRoot (aggregation stage)（聚合阶段）

$replaceRoot with an Embedded Document Field带有嵌入式文档字段

A collection named people contains the following documents:名为people的集合包含以下文档：

{ "_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 $mergeObjects operation. 以下操作使用$replaceRoot阶段将每个输入文档替换为$mergeObjects操作的结果。The $mergeObjects expression merges the specified default document with the pets document.$mergeObjects表达式将指定的默认文档与pets文档合并。

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

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

{ "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 with a Document Nested in an Array文档嵌套在数组中

A collection named students contains the following documents: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 }
      ]
   }
])

The following operation promotes the embedded document(s) with the grade field greater than or equal to 90 to the top level:以下操作将grade字段大于或等于90的嵌入文档提升到顶级：

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

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

{ "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 with a newly created document使用新创建的文档

You can also create new documents as part of the $replaceRoot stage and use them to replace all the other fields.您还可以在$replaceRoot阶段创建新文档，并使用它们替换所有其他字段。

A collection named contacts contains the following documents:名为contacts的集合包含以下文档：

{ "_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" }

The following operation creates a new document out of the first_name and last_name fields.以下操作从first_name和last_name字段创建新文档。

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

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

{ "full_name" : "Gary Sheffield" }
{ "full_name" : "Nancy Walker" }
{ "full_name" : "Peter Sumner" }

$replaceRoot with a New Document Created from $$ROOT and a Default Document使用从$$ROOT创建的新文档和默认文档

Create a collection named contacts with the following documents:使用以下文档创建一个名为contacts的集合：

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 $mergeObjects to output current documents with default values for missing fields:以下操作使用$replaceRoot和$mergeObjects来输出当前文档，并为缺失的字段设置默认值：

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

The aggregation returns the following documents:聚合返回以下文档：

{
  _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'
}