db={"Supplier":[{"_id":ObjectId("5a544fe234602415114601d3"),"GstDetails":[{"_id":ObjectId("5e4837374d62f4c95163908e"),"StateId":"1","GstIn":"33ABFFM1655H1ZF","StateDesc":"TAMIL NADU","CityDesc":"CHENNAI"},{"_id":ObjectId("5e4837484d62f4c9516395e8"),"StateId":"1","GstIn":"33ABFFM1655H1ZF","StateDesc":"TAMIL NADU","CityDesc":"CHENNAI"},{"_id":ObjectId("5e4837484d62f4c9516395e9"),"StateId":"2","GstIn":"XXXXX","StateDesc":"TAMIL NADU","CityDesc":"CHENNAI"}]},{"GstDetails":[{"_id":ObjectId("5e4837374d62f4c95163900e"),"StateId":"3","GstIn":"33ABFFM1655H1ZF","StateDesc":"TAMIL NADU","CityDesc":"CHENNAI"}]}]}

db.Supplier.aggregate([{$unwind:"$GstDetails"},{$group:{_id:{_id:"$_id",StateId:"$GstDetails.StateId"},GstDetails:{$push:"$GstDetails"}}},{$addFields:{GstDetails:{$slice:["$GstDetails",1]}}},{$unwind:"$GstDetails"},{$group:{_id:"$_id._id",GstDetails:{$push:"$GstDetails"}}}])