Template
single collection
multiple collections
mgodatagen
update
indexe
explain
Stage
Database
bson
mgodatagen
[{"key":1},{"key":2},{"key":3}]
Query
db.collection.aggregate([{$group:{_id:null,elem:{$push:"$key"}}},{$project:{elem:{$setDifference:[[1,2,3,4,5,6,7],"$elem"]}}}])
Result