$project (aggregation)

Include Existing Fields包括现有字段

• The _id field is, by default, included in the output documents. 默认情况下，_id字段包含在输出文档中。To include any other fields from the input documents in the output documents, you must explicitly specify the inclusion in $project.要在输出文档中包含输入文档中的任何其他字段，必须在$project中明确指定包含项。
• If you specify an inclusion of a field that does not exist in the document, $project ignores that field inclusion and does not add the field to the document.如果指定包含文档中不存在的字段，$project将忽略该字段包含，并且不会将该字段添加到文档中。

Suppress the _id Field抑制_id字段

By default, the _id field is included in the output documents. 默认情况下，_id字段包含在输出文档中。To exclude the _id field from the output documents, you must explicitly specify the suppression of the _id field in $project.要从输出文档中排除_id字段，必须在$project中明确指定_id字段的抑制。

Exclude Fields排除字段

If you specify the exclusion of a field or fields, all other fields are returned in the output documents.如果指定排除一个或多个字段，则输出文档中将返回所有其他字段。

{ $project: { "<field1>": 0, "<field2>": 0, ... } } // Return all but the specified fields

If you specify the exclusion of a field other than _id, you cannot employ any other $project specification forms: i.e. if you exclude fields, you cannot also specify the inclusion of fields, reset the value of existing fields, or add new fields. 如果指定排除_id以外的字段，则不能使用任何其他$project规范形式：即，如果排除字段，则也不能指定包含字段、重置现有字段的值或添加新字段。This restriction does not apply to conditional exclusion of a field using the REMOVE variable.此限制不适用于使用REMOVE变量对字段进行条件排除。

See also the $unset stage to exclude fields.另请参见$unset阶段以排除字段。

Add New Fields or Reset Existing Fields添加新字段或重置现有字段

To add a new field or to reset the value of an existing field, specify the field name and set its value to some expression. 要添加新字段或重置现有字段的值，请指定字段名称并将其值设置为某个表达式。For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。

Embedded Document Fields嵌入文档字段

When projecting or adding/resetting a field within an embedded document, you can either use dot notation, as in在嵌入文档中投影或添加/重置字段时，可以使用点符号，如

"contact.address.country": <1 or 0 or expression>

Or you can nest the fields:或者可以嵌套字段：

contact: { address: { country: <1 or 0 or expression> } }

When nesting the fields, you cannot use dot notation inside the embedded document to specify the field, e.g. contact: {"address.country": <1 or 0 or expression> } is invalid.嵌套字段时，不能在嵌入文档中使用点符号指定字段，例如contact: {"address.country": <1 or 0 or expression> }无效。

{ $project: { contact: 1, "contact.address.country": 1 } }

{ $project: { "contact.address.country": 1, contact: 1 } }

Restrictions限制

An error is returned if the $project specification is an empty document.如果$project规范为空文档，则返回错误。

You cannot use an array index with the $project stage. 不能将数组索引与$project阶段一起使用。See Array Indexes are Unsupported.请参阅不支持数组索引。

Include Specific Fields in Output Documents在输出文档中包括特定字段

Consider a books collection with the following document:考虑具有以下文档的books集合：

{
  "_id" : 1,
  title: "abc123",
  isbn: "0001122223334",
  author: { last: "zzz", first: "aaa" },
  copies: 5
}

The following $project stage includes only the _id, title, and the author fields in its output documents:以下$project阶段在其输出文档中仅包括_id、title和author字段：

db.books.aggregate( [ { $project : { title : 1 , author : 1 } } ] )

The operation results in the following document:操作结果如下：

{ "_id" : 1, "title" : "abc123", "author" : { "last" : "zzz", "first" : "aaa" } }

Suppress _id Field in the Output Documents在输出文档中抑制_id字段

The _id field is always included by default. 默认情况下，始终包含_id字段。To exclude the _id field from the output documents of the $project stage, specify the exclusion of the _id field by setting it to 0 in the projection document.要从$project阶段的输出文档中排除_id字段，请在投影文档中将_id字段设置为0来指定排除。

Consider a books collection with the following document:考虑具有以下文档的books集合：

{
  "_id" : 1,
  title: "abc123",
  isbn: "0001122223334",
  author: { last: "zzz", first: "aaa" },
  copies: 5
}

The following $project stage excludes the _id field but includes the title, and the author fields in its output documents:以下$project阶段不包括_id字段，但在其输出文档中包括title和author字段：

db.books.aggregate( [ { $project : { _id: 0, title : 1 , author : 1 } } ] )

The operation results in the following document:操作结果如下：

{ "title" : "abc123", "author" : { "last" : "zzz", "first" : "aaa" } }

Exclude Fields from Output Documents从输出文档中排除字段

Consider a books collection with the following document:考虑具有以下文档的books集合：

{
  "_id" : 1,
  title: "abc123",
  isbn: "0001122223334",
  author: { last: "zzz", first: "aaa" },
  copies: 5,
  lastModified: "2016-07-28"
}

The following $project stage excludes the lastModified field from the output:以下$project阶段将lastModified字段从输出中排除：

db.books.aggregate( [ { $project : { "lastModified": 0 } } ] )

See also the $unset stage to exclude fields.另请参见$unset阶段以排除字段。

Exclude Fields from Embedded Documents从嵌入文档中排除字段

Consider a books collection with the following document:考虑具有以下文档的books集合：

{
  "_id" : 1,
  title: "abc123",
  isbn: "0001122223334",
  author: { last: "zzz", first: "aaa" },
  copies: 5,
  lastModified: "2016-07-28"
}

The following $project stage excludes the author.first and lastModified fields from the output:以下$project阶段从输出中排除author.first和lastModified字段：

db.books.aggregate( [ { $project : { "author.first" : 0, "lastModified" : 0 } } ] )

Alternatively, you can nest the exclusion specification in a document:或者，您可以在文档中嵌套排除规范：

db.bookmarks.aggregate( [ { $project: { "author": { "first": 0}, "lastModified" : 0 } } ] )

Both specifications result in the same output:两种规格产生相同的输出：

{
   "_id" : 1,
   "title" : "abc123",
   "isbn" : "0001122223334",
   "author" : {
      "last" : "zzz"
   },
   "copies" : 5,
}

See also the $unset stage to exclude fields.另请参见$unset阶段以排除字段。

Conditionally Exclude Fields有条件排除字段

You can use the variable REMOVE in aggregation expressions to conditionally suppress a field.可以在聚合表达式中使用变量REMOVE有条件地抑制字段。

Consider a books collection with the following document:考虑具有以下文档的books集合：

{
  "_id" : 1,
  title: "abc123",
  isbn: "0001122223334",
  author: { last: "zzz", first: "aaa" },
  copies: 5,
  lastModified: "2016-07-28"
}
{
  "_id" : 2,
  title: "Baked Goods",
  isbn: "9999999999999",
  author: { last: "xyz", first: "abc", middle: "" },
  copies: 2,
  lastModified: "2017-07-21"
}
{
  "_id" : 3,
  title: "Ice Cream Cakes",
  isbn: "8888888888888",
  author: { last: "xyz", first: "abc", middle: "mmm" },
  copies: 5,
  lastModified: "2017-07-22"
}

The following $project stage uses the REMOVE variable to excludes the author.middle field only if it equals "":以下$project阶段仅在等于""时使用REMOVE变量排除author.middle字段：

db.books.aggregate( [
   {
      $project: {
         title: 1,
         "author.first": 1,
         "author.last" : 1,
         "author.middle": {
            $cond: {
               if: { $eq: [ "", "$author.middle" ] },
               then: "$$REMOVE",
               else: "$author.middle"
            }
         }
      }
   }
] )

The aggregation operation results in the following output:聚合操作将产生以下输出：

{ "_id" : 1, "title" : "abc123", "author" : { "last" : "zzz", "first" : "aaa" } }
{ "_id" : 2, "title" : "Baked Goods", "author" : { "last" : "xyz", "first" : "abc" } }
{ "_id" : 3, "title" : "Ice Cream Cakes", "author" : { "last" : "xyz", "first" : "abc", "middle" : "mmm" } }

Include Specific Fields from Embedded Documents包含嵌入文档中的特定字段

Consider a bookmarks collection with the following documents:考虑包含以下文档的bookmarks集合：

{ _id: 1, user: "1234", stop: { title: "book1", author: "xyz", page: 32 } }
{ _id: 2, user: "7890", stop: [ { title: "book2", author: "abc", page: 5 }, { title: "book3", author: "ijk", page: 100 } ] }

To include only the title field in the embedded document in the stop field, you can use the dot notation:要在stop字段中仅包含嵌入文档中的title字段，可以使用点符号：

db.bookmarks.aggregate( [ { $project: { "stop.title": 1 } } ] )

Or, you can nest the inclusion specification in a document:或者，您可以在文档中嵌套包含规范：

db.bookmarks.aggregate( [ { $project: { stop: { title: 1 } } } ] )

Both specifications result in the following documents:这两个规范产生了以下文件：

{ "_id" : 1, "stop" : { "title" : "book1" } }
{ "_id" : 2, "stop" : [ { "title" : "book2" }, { "title" : "book3" } ] }

Include Computed Fields包括计算字段

Consider a books collection with the following document:考虑具有以下文档的books集合：

{
  "_id" : 1,
  title: "abc123",
  isbn: "0001122223334",
  author: { last: "zzz", first: "aaa" },
  copies: 5
}

The following $project stage adds the new fields isbn, lastName, and copiesSold:以下$project阶段添加了新字段isbn、lastName和copiesSold：

db.books.aggregate(
   [
      {
         $project: {
            title: 1,
            isbn: {
               prefix: { $substr: [ "$isbn", 0, 3 ] },
               group: { $substr: [ "$isbn", 3, 2 ] },
               publisher: { $substr: [ "$isbn", 5, 4 ] },
               title: { $substr: [ "$isbn", 9, 3 ] },
               checkDigit: { $substr: [ "$isbn", 12, 1] }
            },
            lastName: "$author.last",
            copiesSold: "$copies"
         }
      }
   ]
)

The operation results in the following document:操作结果如下：

{
   "_id" : 1,
   "title" : "abc123",
   "isbn" : {
      "prefix" : "000",
      "group" : "11",
      "publisher" : "2222",
      "title" : "333",
      "checkDigit" : "4"
   },
   "lastName" : "zzz",
   "copiesSold" : 5
}

Project New Array Fields投影新字段数组

For example, if a collection includes the following document:例如，如果集合包含以下文档：

{ "_id" : ObjectId("55ad167f320c6be244eb3b95"), "x" : 1, "y" : 1 }

The following operation projects the fields x and y as elements in a new field myArray:以下操作将字段x和y投影为新字段myArray中的元素：

db.collection.aggregate( [ { $project: { myArray: [ "$x", "$y" ] } } ] )

The operation returns the following document:运算返回以下文档：

{ "_id" : ObjectId("55ad167f320c6be244eb3b95"), "myArray" : [ 1, 1 ] }

If array specification includes fields that are non-existent in a document, the operation substitutes null as the value for that field.如果数组规范包含文档中不存在的字段，则该操作将null替换为该字段的值。

For example, given the same document as above, the following operation projects the fields x, y, and a non-existing field $someField as elements in a new field myArray:例如，给定与上述相同的文档，以下操作将字段x、y和不存在的字段$someField投影为新字段myArray中的元素：

db.collection.aggregate( [ { $project: { myArray: [ "$x", "$y", "$someField" ] } } ] )

The operation returns the following document:运算返回以下文档：

{ "_id" : ObjectId("55ad167f320c6be244eb3b95"), "myArray" : [ 1, 1, null ] }

Array Indexes are Unsupported不支持数组索引

You cannot use an array index with the $project stage. 不能将数组索引与$project阶段一起使用。This section shows an example.本节显示了一个示例。

Create the following pizzas collection:创建以下pizzas集合：

db.pizzas.insert( [
   { _id: 0, name: [ 'Pepperoni' ] },
] )

The following example returns the pizza:以下示例返回比萨饼：

db.pizzas.aggregate( [
   { $project: { x: '$name', _id: 0 } },
] )

The pizza is returned in the example output:比萨饼在示例输出中返回：

[ { x: [ 'Pepperoni' ] } ]

The following example uses an array index ($name.0) to attempt to return the pizza:以下示例使用数组索引（$name.0）尝试返回比萨饼：

db.pizzas.aggregate( [
   { $project: { x: '$name.0', _id: 0 } },
] )

The pizza is not returned in the example output:示例输出中未返回披萨：

[ { x: [] } ]