$match on sum of two fields

Question

In the mongodb aggregation pipeline how do I match on the sum of two fields being non zero? So I can say the following to give me all the documents where startPos is greater than 10

db.strategy.aggregate(
    [
        {
            $match: {
             "startPos": {"$gt" :10}
            }
        },

    ]
);

But how do I say something like:

db.strategy.aggregate(
    [
        {
            $match: {
             {"$subtract":["endPos","startPos"]}: {"$gt" :10} <== Clearly not legal
            }
        },

    ]
);

Answer 1

You can use redact to achieve this.

db.strategy.aggregate([
                {"$redact" : 
                    { "$cond" : { 
                        if : { $gt: [ {"$subtract":["$endPos","$startPos"] }, 10] }, 
                        then: "$$KEEP", 
                        else: "$$PRUNE" 
                        } 
                    }         
                }
            ] );