[{"sections":[{"desc":"no flow ID","sectionObj":[{"smartFlowIdList":[]},{"smartFlowIdList":[]}]},{"desc":"has flow ID","sectionObj":[{"smartFlowIdList":["smartFlowId1","smartFlowId2"]}]}]},{"sections":[{"desc":"no flow ID","sectionObj":[{"smartFlowIdList":[]},{"smartFlowIdList":[]}]},{"desc":"has flow ID","sectionObj":[{"smartFlowIdList":[]}]}]}]

db.collection.aggregate([{$match:{$expr:{$gt:[{$sum:{$map:{input:"$sections",as:"external",in:{$sum:[{$reduce:{input:"$$external.sectionObj",initialValue:0,in:{$sum:["$$value",{$size:"$$this.smartFlowIdList"}]}}}]}}}},0]}}}])