[{"_id":1,"team":"Anteater",log:[{run:1,distance:8},{run2:2,distance:7.5},{run:3,distance:9.2}]},{"_id":2,"team":"Bears",log:[{run:1,distance:18},{run2:2,distance:17},{run:3,distance:16}]},{"_id":3,"team":"Cobras",log:[{run:1,distance:2}]}]

db.collection.aggregate([{"$addFields":{"firstrun":{"$first":"$log"},"lastrun":{"$last":"$log"}}}])