$match (aggregation)

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Definition

  • $match
    • Filters the documents to pass only the documents that match the specified condition(s) to the next pipeline stage.

The $match stage has the following prototype form:

{ $match: { <query> } }

$match takes a document that specifies the query conditions. The query syntax is identical to the read operation query syntax; i.e. $match does not accept raw aggregation expressions. Instead, use a $expr query expression to include aggregation expression in $match.

Behavior

Pipeline Optimization

Restrictions

{ $match: { $expr: { <aggregation expression> } } }
  • You cannot use $where in $match queries as part of the aggregation pipeline.

  • To use $text in the $match stage, the $match stage has to be the first stage of the pipeline.

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Examples

The examples use a collection named articles with the following documents:

{ "_id" : ObjectId("512bc95fe835e68f199c8686"), "author" : "dave", "score" : 80, "views" : 100 }
{ "_id" : ObjectId("512bc962e835e68f199c8687"), "author" : "dave", "score" : 85, "views" : 521 }
{ "_id" : ObjectId("55f5a192d4bede9ac365b257"), "author" : "ahn", "score" : 60, "views" : 1000 }
{ "_id" : ObjectId("55f5a192d4bede9ac365b258"), "author" : "li", "score" : 55, "views" : 5000 }
{ "_id" : ObjectId("55f5a1d3d4bede9ac365b259"), "author" : "annT", "score" : 60, "views" : 50 }
{ "_id" : ObjectId("55f5a1d3d4bede9ac365b25a"), "author" : "li", "score" : 94, "views" : 999 }
{ "_id" : ObjectId("55f5a1d3d4bede9ac365b25b"), "author" : "ty", "score" : 95, "views" : 1000 }

Equality Match

The following operation uses $match to perform a simple equality match:

db.articles.aggregate(
    [ { $match : { author : "dave" } } ]
);

The $match selects the documents where the author field equals dave , and the aggregation returns the following:

{ "_id" : ObjectId("512bc95fe835e68f199c8686"), "author" : "dave", "score" : 80, "views" : 100 }
{ "_id" : ObjectId("512bc962e835e68f199c8687"), "author" : "dave", "score" : 85, "views" : 521 }

Perform a Count

The following example selects documents to process using the $match pipeline operator and then pipes the results to the $group pipeline operator to compute a count of the documents:

db.articles.aggregate( [
  { $match: { $or: [ { score: { $gt: 70, $lt: 90 } }, { views: { $gte: 1000 } } ] } },
  { $group: { _id: null, count: { $sum: 1 } } }
] );

In the aggregation pipeline, $match selects the documents where either the score is greater than 70 and less than 90 or the views is greater than or equal to 1000 . These documents are then piped to the $group to perform a count. The aggregation returns the following:

{ "_id" : null, "count" : 5 }