$elemMatch (projection)

See also参阅

$elemMatch (query)

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Definition定义

$elemMatch

The $elemMatch operator limits the contents of an <array> field from the query results to contain only the first element matching the $elemMatch condition.$elemMatch运算符将查询结果中<array>字段的内容限制为仅包含与$elemMatch条件匹配的第一个元素。

Usage Considerations使用注意事项

Returned Element返回元素

Both the $ operator and the $elemMatch operator project the first matching element from an array based on a condition.$运算符和$elemMatch运算符都根据条件从数组中投影第一个匹配元素。

The $ operator projects the first matching array element from each document in a collection based on some condition from the query statement.$运算符根据查询语句中的某些条件，从集合中的每个文档中投影第一个匹配的数组元素。

The $elemMatch projection operator takes an explicit condition argument. $elemMatch投影运算符接受一个显式条件参数。This allows you to project based on a condition not in the query, or if you need to project based on multiple fields in the array’s embedded documents. 这允许您基于不在查询中的条件进行投影，或者如果需要基于数组嵌入文档中的多个字段进行投影。See Array Field Limitations for an example.有关示例，请参阅数组字段限制。

Field Order字段排序

Starting in MongoDB 4.4, regardless of the ordering of the fields in the document, the $elemMatch projection of an existing field returns the field after the other existing field inclusions.从MongoDB 4.4开始，无论文档中字段的顺序如何，现有字段的$elemMatch投影都会在其他现有字段包含之后返回该字段。

For example, consider a players collection with the following document:例如，考虑一个具有以下文档的players集合：

db.players.insert( {
   name: "player1",
   games: [ { game: "abc", score: 8 }, { game: "xyz", score: 5 } ],
   joined: new Date("2020-01-01"),
   lastLogin: new Date("2020-05-01")
} )

In version 4.4+, the following projection returns the games field after the other existing fields included in the projection even though in the document, the field is listed before joined and lastLogin fields:在版本4.4+中，以下投影将在投影中包含的其他现有字段之后返回games字段，即使在文档中，该字段在joined和lastLogin字段之前列出：

db.players.find( {}, { games: { $elemMatch: { score: { $gt: 5 } } }, joined: 1, lastLogin: 1 } )

That is, the operation returns the following document:也就是说，该操作返回以下文档：

{
   "_id" : ObjectId("5edef64a1c099fff6b033977"),
   "joined" : ISODate("2020-01-01T00:00:00Z"),
   "lastLogin" : ISODate("2020-05-01T00:00:00Z"),
   "games" : [ { "game" : "abc", "score" : 8 } ]
}

In version 4.2 and earlier, the $elemMatch projection of an existing field upholds the ordering in the document:在4.2版及更早版本中，现有字段的$elemMatch投影支持文档中的顺序：

{
  "_id" : ObjectId("5edef91e76ddff7d92f118e1"),
  "games" : [ { "game" : "abc", "score" : 8 } ],
  "joined" : ISODate("2020-01-01T00:00:00Z"),
  "lastLogin" : ISODate("2020-05-01T00:00:00Z")
}

Restrictions限制

Examples示例

The examples on the $elemMatch projection operator assumes a collection schools with the following documents:$elemMatch投影运算符上的示例假定集合schools包含以下文档：

{
 _id: 1,
 zipcode: "63109",
 students: [
              { name: "john", school: 102, age: 10 },
              { name: "jess", school: 102, age: 11 },
              { name: "jeff", school: 108, age: 15 }
           ]
}
{
 _id: 2,
 zipcode: "63110",
 students: [
              { name: "ajax", school: 100, age: 7 },
              { name: "achilles", school: 100, age: 8 },
           ]
}
{
 _id: 3,
 zipcode: "63109",
 students: [
              { name: "ajax", school: 100, age: 7 },
              { name: "achilles", school: 100, age: 8 },
           ]
}
{
 _id: 4,
 zipcode: "63109",
 students: [
              { name: "barney", school: 102, age: 7 },
              { name: "ruth", school: 102, age: 16 },
           ]
}

$elemMatch with Multiple Fields

The $elemMatch projection can specify criteria on multiple fields:$elemMatch投影可以在多个字段上指定条件：

The following find() operation queries for all documents where the value of the zipcode field is 63109. 下面的find()操作查询zipcode字段值为63109的所有文档。The projection includes the first matching element of the students array where the school field has a value of 102 and the age field is greater than 10:投影包括students数组的first匹配元素，其中school字段的值为102，age字段的值大于10：

db.schools.find( { zipcode: "63109" },
                 { students: { $elemMatch: { school: 102, age: { $gt: 10} } } } )

The operation returns the three documents that have zipcode equal to 63109:该操作返回zipcode等于63109的三个文档：

{ "_id" : 1, "students" : [ { "name" : "jess", "school" : 102, "age" : 11 } ] }
{ "_id" : 3 }
{ "_id" : 4, "students" : [ { "name" : "ruth", "school" : 102, "age" : 16 } ] }

The document with _id equal to 3 does not contain the students field since no array element matched the $elemMatch criteria._id等于3的文档不包含students字段，因为没有与$elemMatch条件匹配的数组元素。

See also参阅

$ (projection) operator