[{_id:ObjectId("5234cc89687ea597eabee675"),code:"xyz",tags:["school","book","bag","headphone","appliance"],qty:[{size:"S",num:10,color:"blue"},{size:"M",num:45,color:"blue"},{size:"L",num:100,color:"green"}]},{_id:ObjectId("5234cc8a687ea597eabee676"),code:"abc",tags:["appliance","school","book"],qty:[{size:"6",num:100,color:"green"},{size:"6",num:70,color:"blue"},{size:"8",num:100,color:"brown"}]},{_id:ObjectId("5234ccb7687ea597eabee677"),code:"efg",tags:["school","book"],qty:[{size:"S",num:10,color:"blue"},{size:"M",num:100,color:"blue"},{size:"L",num:100,color:"green"}]},{_id:ObjectId("52350353b2eff1353b349de9"),code:"ijk",tags:["electronics","school"],qty:[{size:"M",num:100,color:"green"}]}]

db.collection.find({$expr:{$allElementsTrue:{$map:{input:"$qty",as:"q",in:{$gt:["$$q.num",50]}}}}})