Database
[{"_id":{"$oid":"5f37eaaae4102600180078a8"},"Details":"Something A","App":{"$oid":"5f37cec069106932d4710ea6"}},{"_id":{"$oid":"5f3860f10a9db00018f2e2e3"},"Details":"Something B","App":{"$oid":"5f37cec069106932d4710ea6"}},{"_id":{"$oid":"5f3908a7db8fd70018b19942"},"Details":"Something C","App":{"$oid":"5f37cec069106932d4710ea7"}},{"_id":{"$oid":"5f390961db8fd70018b19945"},"Details":"Something D","App":{"$oid":"5f37cec069106932d4710ea8"}}]
Query
db.collection.aggregate([{"$match":{"$expr":{"$eq":["$App",{"$toObjectId":"5f37cec069106932d4710ea8"}]}}}])