Mongodb $project with $multiply return null

Question

I create a $project stage in my Mongodb Compass, in this stage I use $multiply to multiply a datafield with a number as in my screenshot. But why my result always return to null My stage screenshot with null return for TotalScore data field

Answer 1

The aggregation work as pipeline so the result of this projection pipeline will give you hop1score field and you won't be able to access it in same operation. So, Either make one more projection or use cond operator as first argument of multiply operator.