[{"_id":0,"name":"example1","serial":"123456","routes":[{"description":"8989","zone":"front"},{"description":"1221","zone":"back"}]},{"_id":1,"name":"example2","serial":"987654","routes":[{"description":"1515","zone":"front"},{"description":"8989","zone":"side"}]}]

db.collection.find({routes:{$elemMatch:{description:"8989",zone:"front"}}})