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$elemMatch（プロジェクション演算子）

Tip

言語

定義

$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.

使用上の考慮事項

返される要素

Both the $ operator and the $elemMatch operator project the first matching element from an array based on a condition.

$ 演算子は、クエリステートメントの条件に基づいて、コレクション内の各ドキュメントから最初に一致する配列要素を投影します。

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. See Array Field Limitations for an example.

フィールドの順序

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.

たとえば、次のドキュメントを含む players コレクションを考えます。

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")
} )

次のプロジェクションでは、ドキュメントではjoinedフィールドとlastLoginフィールドの前にフィールドがリストされているにもかかわらず、プロジェクションに含まれる他の既存フィールドの後にgamesフィールドが返されます。

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

つまり、この操作は次のドキュメントを返します。

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

制限事項

db.collection.find() operations on views do not support $elemMatch projection operator.
You cannot specify a $text query operator in an $elemMatch.

例

The examples on the $elemMatch projection operator assumes a collection schools with the following documents:

{
 _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 },
           ],
 athletics: [ "swimming", "basketball", "football" ]
},
{
 _id: 3,
 zipcode: "63109",
 students: [
              { name: "ajax", school: 100, age: 7 },
              { name: "achilles", school: 100, age: 8 },
           ],
 athletics: [ "baseball", "basketball", "soccer" ]
},
{
 _id: 4,
 zipcode: "63109",
 students: [
              { name: "barney", school: 102, age: 7 },
              { name: "ruth", school: 102, age: 16 },
           ]
}

郵便番号検索

The operation returns the following documents that have a zipcode value of "63109" and projects the students array using $elemMatch:

{ "_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. However, the $elemMatch projection returns only the first matching element from the array.
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.

`$elemMatch` 複数のフィールド付き

The $elemMatch projection can specify criteria on multiple fields.

この操作では、次のように、zipcode の値が "63109" である 3 つのドキュメントが返されます。

{ "_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.

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. For example, $elemMatch attempts to match objects, instead of scalar values, within the array for the following in the projection:

上記の例では、zipcode 値が "63109" であるドキュメントが返されますが、プロジェクション操作では一致する要素が見つからなかったため、これらのドキュメントには _idフィールドのみが含まれます。

この操作では、zipcode の値が "63109" である 3 つのドキュメントが返されます。返されるドキュメントには、_id フィールドと athletics 配列の一致する要素（存在する場合）が含まれます。

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

The document with _id equal to 3 is the only document that matched the $elemMatch criteria.

Tip

$ (projection) 演算子

戻る

$slice