$elemMatch (query)¶

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Definition
Behavior
Examples

参见

Definition¶

$elemMatch¶

The $elemMatch operator matches documents that contain an array field with at least one element that matches all the specified query criteria.

{ <field>: { $elemMatch: { <query1>, <query2>, ... } } }

If you specify only a single <query> condition in the $elemMatch expression, you do not need to use $elemMatch.

Behavior¶

You cannot specify a $where expression as a query criterion for $elemMatch.

Examples¶

Element Match¶

Given the following documents in the scores collection:

{ _id: 1, results: [ 82, 85, 88 ] }
{ _id: 2, results: [ 75, 88, 89 ] }

The following query matches only those documents where the results array contains at least one element that is both greater than or equal to 80 and is less than 85.

db.scores.find(
   { results: { $elemMatch: { $gte: 80, $lt: 85 } } }
)

The query returns the following document since the element 82 is both greater than or equal to 80 and is less than 85

{ "_id" : 1, "results" : [ 82, 85, 88 ] }

For more information on specifying multiple criteria on array elements, see 指定数组元素的多个查询条件.

Array of Embedded Documents¶

Given the following documents in the survey collection:

{ _id: 1, results: [ { product: "abc", score: 10 }, { product: "xyz", score: 5 } ] }
{ _id: 2, results: [ { product: "abc", score: 8 }, { product: "xyz", score: 7 } ] }
{ _id: 3, results: [ { product: "abc", score: 7 }, { product: "xyz", score: 8 } ] }

The following query matches only those documents where the results array contains at least one element with both product equal to "xyz" and score greater than or equal to 8.

db.survey.find(
   { results: { $elemMatch: { product: "xyz", score: { $gte: 8 } } } }
)

Specifically, the query matches the following document:

{ "_id" : 3, "results" : [ { "product" : "abc", "score" : 7 }, { "product" : "xyz", "score" : 8 } ] }

Single Query Condition¶

If you specify a single query predicate in the $elemMatch expression, $elemMatch is not necessary.

For example, consider the following example where $elemMatch specifies only a single query predicate { product: "xyz" }:

db.survey.find(
   { results: { $elemMatch: { product: "xyz" } } }
)

Since the $elemMatch only specifies a single condition, the $elemMatch expression is not necessary, and instead you can use the following query:

db.survey.find(
   { "results.product": "xyz" }
)

For more information on querying arrays, see 数组上的查询, including 指定数组元素的多个查询条件 and 嵌入文档数组 sections.

← $all $size →