[{_id:"fgsergehegoieofgesfglesfg",user:"dfjksjdnfkjsdfkjsdklfjglkbj",caption:"This is the post caption",action:[{actionName:"e1",user:"sdfasdsdfasdfdsdfac951e5c"},{actionName:"e1",user:"asdfmadfadfee103c9c951e5d"},{actionName:"e2",user:"op34937cdbae0cd4160bbec"},{actionName:"e2",user:"2543ebbasdfd1750690b5b01c"},{actionName:"e3",user:"asdfcfebdb5dd1750690b5b01d"},],shares:["ewrebdb5ddadsf5069sadf1d","asdfsdfbb85dd1750690b5b01c","fasec92dsfasde103c9c95df5d"]}]

db.collection.aggregate([{$unwind:"$action"},{$group:{_id:{_id:"$_id",actionName:"$action.actionName"},user:{$first:"$user"},caption:{$first:"$caption"},count:{$sum:1},shareCount:{$first:{$size:"$shares"}}}},{$group:{_id:"$_id._id",user:{$first:"$user"},caption:{$first:"$caption"},shareCount:{$first:"$shareCount"},actionCount:{$push:{actionName:"$_id.actionName",count:"$count"}}}}])