Template
single collection
multiple collections
mgodatagen
update
indexe
explain
Stage
Database
bson
mgodatagen
[{"a":1,"b":2,"c":3},{"a":9,"b":12},{"a":5}]
Query
db.collection.aggregate([{$addFields:{count:{$size:{$objectToArray:"$$ROOT"}}}},{$match:{count:{$eq:4}}},{$project:{count:0}}])
Result