Database
[{school:[{student:"raj",subjects:["P","C","M"],std:10},{student:"ram",subjects:[],std:8},{student:"rahul",subjects:["P"],std:9}]}]
Query
db.collection.aggregate([{$project:{school:{$map:{input:"$school",in:{student:"$$this.student",subjects:{$cond:[{$eq:[0,{$size:{$ifNull:["$$this.subjects",[]]}}]},"$$REMOVE","$$this.subjects"]}}}}}}])