Database
[{action:"SEND_ACTION_START",data:{stuff:{delivered:{a:1,b:2,c:3}}}},{action:"SEND_ACTION",data:{stuff:{delivered:[{0:{a:1,b:2,c:3}}]}}},{action:"SEND",data:{stuff:{delivered:[{0:{a:1,b:2,c:3}},{1:{a:11,b:22,c:33}}],marker:1}}}]
Query
db.collection.aggregate([{$match:{action:"SEND",}},{$replaceRoot:{newRoot:{$arrayElemAt:["$data.stuff.delivered",{$toInt:"$data.stuff.marker"}]}}}])