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Query and Projection Operators

Projection Operators

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Query and Projection Operators

Projection Operators

$elemMatch (projection)

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$elemMatch (query)

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

Returned Element

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

The $ operator projects the first matching array element from each document in a collection 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.

Field Order

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.

For example, consider a players collection with the following document:

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:

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

That is, the operation returns the following document:

{
   "_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:

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

Restrictions

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

Examples

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 },
           ]
}
{
 _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 that have zipcode equal to 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` 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.

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