[{_id:ObjectId("64103dd44c7b1355e4812e87"),categoryId:2,categoryTypes:{1:"hello world",2:"something else",3:"still more stuff"}},{_id:ObjectId("64103dd44c7b1355e4812e88"),categoryId:4,categoryTypes:{1:"hello world",2:"something else",3:"still more stuff"}}]

db.collection.aggregate([{$set:{allKeys:{$objectToArray:"$categoryTypes"}}},{$match:{$expr:{$in:[{$toString:"$categoryId"},"$allKeys.k"]}}},{$unset:"allKeys"}])