[{"_id":ObjectId("571c5c87faf473f40fd0317c"),"name":"test 1","sections":[{"Id":1,"name":"first section"},{"Id":2,"name":"section 2"},{"Id":3,"name":"section 3"}]}]

db.collection.aggregate([{"$project":{"maxSectionId":{"$arrayElemAt":["$sections",{"$indexOfArray":["$sections.Id",{"$max":"$sections.Id"}]}]}}}])