Database
db={"user":[{"_id":ObjectId("5fd6fa3b4150e552f0b75935"),"name":"Bobby","usertype":"parent","studententity":[{"usertype":"student","_id":"5f8a6d0900e0030c7031d56a","name":"Adya"},{"usertype":"student","_id":"5f9bb96fdf44b931b40c6043","name":"Aruja"}]}],"transaction":[{"_id":ObjectId("5fd6fa3b4150e552f0b75936"),ref_id:ObjectId("5fd6fa3b4150e552f0b75935")}]}
Query
db.user.aggregate([{"$lookup":{"from":"transaction","localField":"_id","foreignField":"ref_id","as":"transactionflag"}},{"$addFields":{"transactionflag":{$toBool:{$size:"$transactionflag"}}}}])