MongoDB divide returns null when dividing by other variable

Question

I am trying to compute some numbers, but MongoDB returns null when using previous computed variable in $divide operator

This is the query:

db.apps.aggregate(

{$project : {
    a: {$multiply : [2,2]},
    b: {$divide: [5, "$a"]},
    c: {$divide: [5, 4]}

}})

Why "b" is null in the result:

/* 0 */
{
    "result" : [ 
        {
            "_id" : ObjectId("5361173d93861f6e5239714e"),
            "a" : 4,
            "b" : null,
            "c" : 1.25
        }, 
        {
            "_id" : ObjectId("536192c1938652d039fa0051"),
            "a" : 4,
            "b" : null,
            "c" : 1.25
        }
    ],
    "ok" : 1
}

EDIT: Thanks to Neil comment, the solution is to have second stage $project

db.apps.aggregate(

{$project : {
    a: {$multiply : [2,2]},
    b: {$divide: [5, "$a"]},
    c: {$divide: [5, 4]}

}},


{$project : {
    a: 1, c:1,
    b: {$divide: [5, "$a"]},   
}}

)

Answer 1

The variables in a $project step are always the fields of the input document of that step. You can not yet access any values computed in the same step. But you can access any computed values in a following $project step. So you can break your computation into two $project-steps like this:

db.apps.aggregate(
    [
        {   $project : {
                a: { $multiply : [2, 2] }
            }
        },
        {   $project : {
                a: 1,
                b: { $divide: [5, "$a"] }   
            }
        }
    ]
);

Answer 2

now you can use let. Like that:

db.apps.aggregate([{
    $project: {
        b: {
            $let: {
                vars: {
                    "a": {$multiply: [2,2]}
                },
                in: {$divide: [5, "$$a"]}
            }
        }
    }}
]);