Database
[{_id:"adklkj389723jk23KLJjl2LU92kJO387","impressions":[{"_id":ObjectId("5b74799535f2722494075981"),"country":"GB","impression_count":22,"_campaignid":ObjectId("5b72d78847db422040ee60cf"),"date":ISODate("2018-08-15T19:00:00Z")},{"_id":ObjectId("5b74799d35f2722494075982"),"country":"GB","impression_count":22,"_campaignid":ObjectId("5b72d7bf47db422040ee60d1"),"date":ISODate("2018-08-15T19:00:00Z")},{"_id":ObjectId("5b7479a735f2722494075983"),"country":"GB","impression_count":20,"_campaignid":ObjectId("5b72d79e47db422040ee60d0"),"date":ISODate("2018-08-15T19:00:00Z")}]}]
Query
db.collection.aggregate([{"$unwind":"$impressions"},{$group:{_id:"$impressions.date",count:{$sum:"$impressions.impression_count"},}}])