db={training_sample:[{_id:ObjectId("56e9b39c732b6122f878f882"),airline:{id:3778,name:"Oman Air",alias:"WY",iata:"OMA"},src_airport:"LHR",dst_airport:"MCT",codeshare:"",stops:0,airplane:333},{_id:ObjectId("56e9b39c732b6122f878f883"),airline:{id:3778,name:"Oman Air",alias:"WY",iata:"OMA"},src_airport:"LHP",dst_airport:"MCT",codeshare:"",stops:0,airplane:354},{_id:ObjectId("56e9b39c732b6122f878f884"),airline:{id:3778,name:"Oman Air",alias:"WY",iata:"OMA"},src_airport:"MCT",dst_airport:"LHP",codeshare:"",stops:0,airplane:235}]}

db.training_sample.aggregate([{$lookup:{from:"training_sample",let:{src_airport:"$src_airport",dst_airport:"$dst_airport"},pipeline:[{$match:{$expr:{$and:[{$eq:["$src_airport","$$dst_airport"]},{$eq:["$$src_airport","$dst_airport"]}]}}}],as:"opposite"}},{$match:{"opposite.0":{$exists:false}}}])