Redact in mongodb seems obscure to me

Question

I'm fighting with redact right now and I'm not sure to understand it.

I just read the docs and tried to use redact on a collection grades (it comes from mongodb online training)

A document in the collection "grades" looks like this :

{
    "_id" : ObjectId("50b59cd75bed76f46522c34e"),
    "student_id" : 0,
    "class_id" : 2,
    "scores" : [ 
        {
            "type" : "exam",
            "score" : 57.92947112575566
        }, 
        {
            "type" : "quiz",
            "score" : 21.24542588206755
        }, 
        {
            "type" : "homework",
            "score" : 68.19567810587429
        }, 
        {
            "type" : "homework",
            "score" : 67.95019716560351
        }, 
        {
            "type" : "homework",
            "score" : 18.81037253352722
        }
    ]
}

I use the following query :

db.grades.aggregate([
    { $match: { student_id: 0 } },
    { 
        $redact: {
            $cond: {
                if: { $eq: [ "$type" , "exam" ] },
                then: "$$PRUNE",
                else: "$$DESCEND"
            }
        }
    }

] );

With this query, each type an exam is found, this sub document should be excluded. And it works, the result is:

{
    "_id" : ObjectId("50b59cd75bed76f46522c34e"),
    "student_id" : 0,
    "class_id" : 2,
    "scores" : [ 
    {
        "type" : "quiz",
        "score" : 21.24542588206755
    }, 
    {
        "type" : "homework",
        "score" : 68.19567810587429
    }, 
    {
        "type" : "homework",
        "score" : 67.95019716560351
    }, 
    {
        "type" : "homework",
        "score" : 18.81037253352722
    }
]
}

but if I invert the condition, I expect that only exams are kept in the result :

if: { $eq: [ "$type" , "exam" ] },
       then: "$$DESCEND",
       else: "$$PRUNE" 

however the result is empty.

I don't understand why subdocument of type "exam" are not included.

Answer 1

The $redact stage starts at the root document and its fields, and only when that document fulfills the condition to $$DESCEND, it examines the sub-documents included in that document. That means the first thing $redact does with your document is examine this:

{
    "_id" : ObjectId("50b59cd75bed76f46522c34e"),
    "student_id" : 0,
    "class_id" : 2,
    "scores" : [] // Some array. I will look at this later.
}

It doesn't even find a type field here, so $eq: [ "$type" , "exam" ] is false. What did you tell $redact to do when the condition is false? else: "$$PRUNE", so the whole document is pruned before the sub-documents are examined.

As a workaround, test if $type is either "exam" or doesn't exist. You didn't explicitly ask for a working solution, so I will leave it as an exercise to you to figure out how to do this.