How to count Booleans in MongoDB with aggregation framework

Question

Documents are stored with the following fields:

_id:
NumberofChildren: Integer
OwnsAHome: Boolean
Income: Integer

I need to use the aggregation framework to sort by number of children so the output is something like this:

Number of Children: 3
Number of People: some value, say 17
Number of People who own a home: some value less than 17 which is a sum of the number of true Booleans
Average income: some value

How would I do this in MongoDB with aggregation , especially in regards to counting the number of times the Boolean OwnsAHome, is true?

Thanks!

Answer 1

The $project phase is your friend in the pipeline by allowing you to create new fields which have different types and values than original fields.

Consider this projection which uses $cond to use one value when something is true and another one when it's false:

{ $project : { numWhoOwnHome : { $cond : [ "$OwnsAHome", 1, 0 ] } } }

If you now do a $group with {$sum : "$numWhoOwnHome"} your result will be the number of people who had OwnsAHome set to true.