$elemMatch (projection)（投影）

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运算符都根据条件投影数组中的第一个匹配元素。

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. 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. $elemMatch投影运算符接受显式条件参数。这允许您根据查询中没有的条件进行投影，或者如果您需要根据数组嵌入文档中的多个字段进行投影。See Array Field Limitations for an example.示例请参见数组字段限制。

Field Order字段顺序

Regardless of the ordering of the fields in the document, the $elemMatch projection of an existing field returns the field after the other existing field inclusions.无论文档中字段的顺序如何，现有字段的$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.insertOne( {
   name: "player1",
   games: [ { game: "abc", score: 8 }, { game: "xyz", score: 5 } ],
   joined: new Date("2020-01-01"),
   lastLogin: new Date("2020-05-01")
} )

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:以下投影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，该字段列在joind和lastLogin字段之前：

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 } ]
}

Restrictions限制

• db.collection.find() operations on views do not support $elemMatch projection operator.视图上的db.collection.find()操作不支持$elemMatch投影运算符。
• You cannot specify a $text query operator in an $elemMatch.不能在$elemMatch中指定$text查询运算符。

Zipcode Search邮政编码搜索

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

 var results = schoolsCollection
     .Find(s => s.ZipCode == "63109")
     .Project(Builders<School>.Projection.ElemMatch(
         field: school => school.Students,
         filter: student => student.School == 102
    )
 ).ToList();

The operation returns the following documents that have a zipcode value of "63109" and projects the students array using $elemMatch:该操作返回以下zipcode值为"63109"的文档，并使用$elemMatch投影students数组：

{ "_id" : 1, "students" : [ { "name" : "john", "school" : 102, "age" : 10 } ] }
{ "_id" : 3 }
{ "_id" : 4, "students" : [ { "name" : "barney", "school" : 102, "age" : 7 } ] }

• For the document with _id equal to 1, the students array contains multiple elements with the school field equal to 102. 对于_id等于1的文档，students数组包含多个school字段等于102的元素。However, the $elemMatch projection returns only the first matching element from the array.但是，$elemMatch投影只返回数组中的第一个匹配元素。
• The document with _id equal to 3 does not contain the students field in the result since no element in its students array matched the $elemMatch condition._id等于3的文档在结果中不包含students字段，因为其students数组中没有元素符合$elemMatch条件。

$elemMatch with Multiple Fields具有多个字段

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

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

var results = schoolsCollection
    .Find(s => s.ZipCode == "63109")
    .Project(Builders<School>.Projection.ElemMatch(
            field: school => school.Students,
            filter: student => (student.School == 102) && (student.Age > 10) 
        )
    ).ToList();

The operation returns the three documents that have a zipcode value of "63109":该操作返回三个zipcode值为"63109"的文档：

{ "_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 $elemMatch criteria._id等于3的文档不包含students字段，因为没有数组元素符合$elemMatch标准。

The argument to $elemMatch matches elements of the array that $elemMatch is projecting. If you specify an equality with a field name to $elemMatch, it attempts to match objects within the array. $elemMatch的参数与$elemMack正在投影的数组元素相匹配。如果将字段名的相等项指定为$elemMatch，它将尝试匹配数组中的对象。For example, $elemMatch attempts to match objects, instead of scalar values, within the array for the following in the projection:例如，$elemMatch尝试匹配投影中以下对象的数组中的对象，而不是标量值：

db.schools.find( { zipcode: "63109" },
                { athletics: { $elemMatch: { athletics: "basketball" } } })

var results = schoolsCollection
    .Find(s => s.ZipCode == "63109")
    .Project(Builders<School>.Projection.ElemMatch(
            "athletics",
            Builders<School>.Filter.Eq("athletics", "basketball"))
    ).ToList();

The preceding examples return the documents that have a zipcode value of "63109", but these documents include only the _id field because the projection operation found no matching elements.前面的示例返回了zipcode值为"63109"的文档，但这些文档仅包含_id字段，因为投影操作没有找到匹配的元素。

db.schools.find( { zipcode: "63109" },
                { athletics: { $elemMatch: { $eq: "basketball" } } })

var results = schoolsCollection
    .Find(s => s.ZipCode == "63109")
    .Project(Builders<School>.Projection.ElemMatch(
        field: "athletics",
        filter: Builders<School>.Filter.Eq("$eq", "basketball"))
    ).ToList();

The operation returns the three documents that have zipcode value of "63109". The returned documents include the _id field and matching elements of the athletics array, if any.该操作返回zipcode值为"63109"的三个文档。返回的文档包括_id字段和田径数组的匹配元素（如果有的话）。

[
   { _id: 1 },
   { _id: 3, athletics: [ 'basketball' ] },
   { _id: 4 }
]

The document with _id equal to 3 is the only document that matched the $elemMatch criteria._id等于3的文档是唯一符合$elemMatch条件的文档。