db={"users":[{"_id":"1","StravaConnect":{"AthleteID":123},"FirstName":"first_name1","LastName":"last_name1"},{"_id":"2","StravaConnect":{"AthleteID":456},"FirstName":"first_name2","LastName":"last_name2"}],"rides":[{"_id":"r1","AthleteID":123,"Length":2}]}

db.users.aggregate([{"$lookup":{"from":"rides","localField":"StravaConnect.AthleteID","foreignField":"AthleteID","as":"rides"}},{"$match":{$expr:{"$gt":[{"$size":"$rides"},0]}}},{"$project":{"FirstName":1,"LastName":1}}])