Template
single collection
multiple collections
mgodatagen
update
indexe
explain
Stage
Database
bson
mgodatagen
[{"key":null},{"key":""},{"A":"a"}]
Query
db.collection.find({},{key:1,status:{$cond:{if:{$or:[{$eq:[{$type:"$key"},"missing"]},{$in:["$key",["",null]]}]},then:[],else:"$key"}}})
Result