$ (projection)

Syntax语法

To return the first array element that matches the specified query condition on the array:要返回与数组上指定查询条件匹配的第一个数组元素，请执行以下操作：

db.collection.find( { <array>: <condition> ... },
                    { "<array>.$": 1 } )
db.collection.find( { <array.field>: <condition> ...},
                    { "<array>.$": 1 } )

db.collection.find( { <someOtherArray>: <condition> ... },
                    { "<array>.$" : 1 } )

Array Field Limitations数组字段限制

MongoDB requires the following when dealing with projection over arrays:MongoDB在处理数组投影时需要以下内容：

• Only one positional $ operator may appear in the projection document.projection文档中只能出现一个位置$运算符。
• Only one array field should appear in the query document. Additional array fields in the query document may lead to undefined behavior.query文档中只应显示一个数组字段。查询文档中的其他数组字段可能会导致未定义的行为。

  For example, the following projection may lead to undefined behavior:例如，以下投影可能导致未定义的行为：

  db.collection.find( { <array>: <value>, <someOtherArray>: <value2> },
                      { "<array>.$": 1 } )

• The query document should only contain a single condition on the array field to which it is applied. query文档应仅在应用它的数组字段上包含一个条件。Multiple conditions may override each other internally and lead to undefined behavior.多个条件可能在内部相互覆盖，并导致未定义的行为。

  To specify criteria on multiple fields of documents inside that array, use the $elemMatch query operator. 要在该数组中的多个文档字段上指定条件，请使用$elemMatch查询运算符。The following query returns the first document inside a grades array that has a mean of greater than 70 and a grade of greater than 90.以下查询返回mean值大于70且grade大于90的grades数组中的第一个文档。

  db.students.find( { grades: { $elemMatch: {
                                              mean: { $gt: 70 },
                                              grade: { $gt:90 }
                                            } } },
                    { "grades.$": 1 } )

  You must use the $elemMatch operator if you need separate conditions for selecting documents and for choosing fields within those documents.如果需要单独的条件来选择文档和在这些文档中选择字段，则必须使用$elemMatch运算符。

Sorts and the Positional Operator排序和位置运算符

When the find() method includes a sort(), the find() method applies the sort() to order the matching documents before it applies the positional $ projection operator.当find()方法包含sort()时，find()方法适用于sort()从而在它应用位置$投影运算符之前排序匹配的文档。

If an array field contains multiple documents with the same field name and the find() method includes a sort() on that repeating field, the returned documents may not reflect the sort order because the sort was applied to the elements of the array before the $ projection operator.如果一个数组字段包含多个具有相同字段名称的文档，并且find()方法在该重复字段上包含sort()，则返回的文档可能不会反映排序顺序，因为排序是在$投影运算符之前应用于数组元素的。

Positional Operator Placement Restriction位置运算符安置限制

Starting in MongoDB 4.4, the $ projection operator can only appear at the end of the field path, for example "field.$" or "fieldA.fieldB.$".从MongoDB 4.4开始，$投影运算符只能出现在字段路径的末尾，例如"field.$"或"fieldA.fieldB.$"。

For example, starting in MongoDB 4.4, the following operation is invalid:例如，在MongoDB 4.4中启动时，以下操作无效：

db.inventory.find( { }, { "instock.$.qty": 1 } ) // Invalid starting in 4.4

To resolve, remove the component of the field path that follows the $ projection operator.若要解决此问题，请删除$投影运算符后面的字段路径的组件。

In previous versions, MongoDB ignores the part of the path that follows the $; i.e. the projection is treated as "instock.$".在以前的版本中，MongoDB会忽略$；即投影被视为"instock.$"。

Positional Operator and $slice Restriction位置运算符和$slice限制

Starting in MongoDB 4.4, find and findAndModify projection cannot include $slice projection expression as part of a $ projection expression.从MongoDB 4.4开始，find和findAndModify投影不能包含$slice投影表达式作为$投影表达式的一部分。

For example, starting in MongoDB 4.4, the following operation is invalid:例如，在MongoDB 4.4中启动时，以下操作无效：

db.inventory.find( { "instock.qty": { $gt: 25 } }, { "instock.$": { $slice: 1 } } ) // Invalid starting in 4.4

In previous versions, MongoDB returns the first element (instock.$) in the instock array that matches the query condition; i.e. the positional projection "instock.$" takes precedence and the $slice:1 is a no-op. 在以前的版本中，MongoDB返回instock数组中与查询条件匹配的第一个元素（instock.$）；即，位置投影"instock.$"优先，$slice:1是no-op。The "instock.$": { $slice: 1 } does not exclude any other document field."instock.$": { $slice: 1 }不排除任何其他文档字段。

Project Array Values投影数组值

A collection students contains the following documents:students集合包含以下文档：

{ "_id" : 1, "semester" : 1, "grades" : [ 70, 87, 90 ] }
{ "_id" : 2, "semester" : 1, "grades" : [ 90, 88, 92 ] }
{ "_id" : 3, "semester" : 1, "grades" : [ 85, 100, 90 ] }
{ "_id" : 4, "semester" : 2, "grades" : [ 79, 85, 80 ] }
{ "_id" : 5, "semester" : 2, "grades" : [ 88, 88, 92 ] }
{ "_id" : 6, "semester" : 2, "grades" : [ 95, 90, 96 ] }

In the following query, the projection { "grades.$": 1 } returns only the first element greater than or equal to 85 for the grades field.在以下查询中，投影{ "grades.$": 1 }只返回grades字段的第一个大于或等于85的元素。

db.students.find( { semester: 1, grades: { $gte: 85 } },
                  { "grades.$": 1 } )

The operation returns the following documents:该操作返回以下文档：

{ "_id" : 1, "grades" : [ 87 ] }
{ "_id" : 2, "grades" : [ 90 ] }
{ "_id" : 3, "grades" : [ 85 ] }

Although the array field grades may contain multiple elements that are greater than or equal to 85, the $ projection operator returns only the first matching element from the array.尽管数组字段grades可以包含大于或等于85的多个元素，$投影运算符仅返回数组中的第一个匹配元素。

Project Array Documents投影数组文档

A students collection contains the following documents where the grades field is an array of documents; each document contain the three field names grade, mean, and std:students集合包含以下文档，其中grades字段是文档的数组；每个文档包含三个字段名称grade、mean和std：

{ "_id" : 7, semester: 3, "grades" : [ { grade: 80, mean: 75, std: 8 },
                                       { grade: 85, mean: 90, std: 5 },
                                       { grade: 90, mean: 85, std: 3 } ] }

{ "_id" : 8, semester: 3, "grades" : [ { grade: 92, mean: 88, std: 8 },
                                       { grade: 78, mean: 90, std: 5 },
                                       { grade: 88, mean: 85, std: 3 } ] }

In the following query, the projection { "grades.$": 1 } returns only the first element with the mean greater than 70 for the grades field:在以下查询中，投影{ "grades.$": 1 }只返回grades字段的第一个mean大于70的元素：

db.students.find(
   { "grades.mean": { $gt: 70 } },
   { "grades.$": 1 }
)

The operation returns the following documents:该操作返回以下文档：

{ "_id" : 7, "grades" : [  {  "grade" : 80,  "mean" : 75,  "std" : 8 } ] }
{ "_id" : 8, "grades" : [  {  "grade" : 92,  "mean" : 88,  "std" : 8 } ] }