Match

Tags:

mongo match docs
Match is to filter docs.
These should come early in the pipes.
Match is vital.
Can be used multiple times. Match is MORE LIKE a FILTER than a FIND.
Putting match at the beginning of a pipe allows the use of indexes, and the passing of indexed docs to the following pipeline parts.

let noStars = { $match: { type: { $ne: 'Star' } } };
db.solarSystem.aggregate([noStars]);

// will return same results as...
db.solarSystem.find({ type: { $ne: 'Star' } });

Match uses mongodb read operation query syntax.

Cannot

use the $where
use projection in $match

must

use the $match as the first operator when using the $text operator

connecting directly to the aggregation db in the atlas cluster

mongo "mongodb://cluster0-shard-00-00-jxeqq.mongodb.net:27017,cluster0-shard-00-01-jxeqq.mongodb.net:27017,cluster0-shard-00-02-jxeqq.mongodb.net:27017/aggregations?replicaSet=Cluster0-shard-0" --authenticationDatabase admin --ssl -u m121 -p aggregations --norc

Examples

Match vs count

// get the count of items that are not a star
db.solarSystem.count({type: ${ne: "Star"}})
// returns "8"

// sort-of-same query, with $match agg
let noStars = { $match: { type: {$ne: "Star"}} }
let countPlanets = { $count: "planets" }

db.solarSystem.aggregate([ noStars, countPlanets ])
// returns {count: 8}