Template
single collection
multiple collections
mgodatagen
update
indexe
explain
Stage
Database
bson
mgodatagen
db={"Test":[{"x":"a"},{"x":"a","y":1},{"x":"a","y":2}]}
Query
db.Test.aggregate([{"$group":{"_id":{"x":"$x"},"y":{"$addToSet":"$y"}}}])
Result