Database
[{"_id":"5a9433fee5878a000f7cba09","trendingStats":{"trendingArr":[],"trendingScore":0}},{"_id":"5a942f79e5878a000f7cba06","trendingStats":{"trendingArr":[],"trendingScore":0}}]
Query
db.collection.aggregate([{$group:{_id:null,data:{$push:{k:"$_id",v:"$trendingStats"}}}},{$replaceRoot:{newRoot:{$arrayToObject:"$data"}}}])