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$elemMatch (projection)¶

参见

$elemMatch (query)

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Definition
Usage Considerations
Examples

Definition¶

$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.

Usage Considerations¶

Both the $ operator and the $elemMatch operator project a subset of elements from an array based on a condition.

The $ operator projects the array elements based on some condition from the query statement.

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.

db.collection.find() operations on views do not support $elemMatch projection operator.

Examples¶

The examples on the $elemMatch projection operator assumes a collection school 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 },
           ]
}
{
 _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 },
           ]
}

Zipcode Search¶

The following find() operation queries for all documents where the value of the zipcode field is 63109. The $elemMatch projection returns only the first matching element of the students array where the school field has a value of 102:

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

The operation returns the following documents:

{ "_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` with Multiple Fields¶

The $elemMatch projection can specify criteria on multiple fields:

The following find() operation queries for all documents where the value of the zipcode field is 63109. 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:

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

The operation returns the three documents that have zipcode equal to 63109:

{ "_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.

参见

$ (projection) operator

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