$elemMatch (query)

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 Specify Multiple Conditions for Array Elements.

Array of Embedded Documents

This statement inserts documents into the survey collection:

db.survey.insertMany( [
   { "_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 } ] },
   { "_id": 4, "results": [ { "product": "abc", "score": 7 },
                            { "product": "def", "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, and are not using the $not or $ne operators inside of $elemMatch, $elemMatch can be omitted.

The following examples return the same documents.

With $elemMatch:

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

Without $elemMatch:

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

However, if your $elemMatch expression contains the $not or $ne operators then omitting the $elemMatch expression changes the documents returned.

The following examples return different documents.

With $elemMatch:

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

Without $elemMatch:

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

With $elemMatch, the first query returns these documents:

{ "_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 } ] }
{ "_id" : 4, "results" : [ { "product" : "abc", "score" : 7 },
                           { "product" : "def", "score" : 8 } ] }

Without $elemMatch, the second query returns this document:

{ "_id" : 4, "results" : [ { "product" : "abc", "score" : 7 },
                           { "product" : "def", "score" : 8 } ] }

The first query returns the documents where any product in the results array is not "xyz". The second query returns the documents where all of the products in the results array are not "xyz".