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$elemMatch (projection operator)

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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

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.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:

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 } ]
}

Restrictions

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

Zipcode Search

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` with Multiple Fields

The $elemMatch projection can specify criteria on multiple fields.

The operation returns the three documents that have a zipcode value of "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.

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:

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.

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.

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

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

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$ (projection) operator

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