Dao De Code

ReactiveMongo, macros and NoSuchElementException: None.get

ReactiveMongo employs scala macros to generate readers(deserializers) and writers(serializers) for scala case classes. They work pretty good, but sometimes you can get a java.lang.NoSuchElementException with message None.get :(

Let's say you have

case class Person(fisrtName: String, lastName: String, age: Int)

object readers {  
  implicit val personReader = Macros.reader[Person]

and somewhere in your code you use it

import readers._  
// ...
def findByFirstName(fName: String) =  
  personCollection.find(BSONDocument("firstName" -> fName)).one[Person]

if your data is correct you are good, but let's say you have some corrupted documents, for example Joe doesn't have last name

> db.person.find().pretty()
    "_id" : ObjectId("526ecf6f7e04ab5f2d1a12ba"),
    "firstName" : "Eugene",
    "lastName" : "Platonov",
    "age" : 27
    "_id" : ObjectId("526ecf817e04ab5f2d1a12bb"),
    "firstName" : "Joe",
    "age" : 23

findByFirstName("Eugene") will return Future of Success, but findByFirstName("Joe") will return you Future of Failure with ugly java.util.NoSuchElementException: None.get which points to the line where your macro-generated reader is defined.

Here is another, even more interesting, but much harder to spot problem:

If you insert your person doc via mongo shell

 db.person.insert({firstName: "Eugene", lastName: "Platonov", age: 27})

the type of age field will be ... right, Double.
Despite that mongo shell will show you "age" : 27 when you search for that document. And again reactivemongo will throw a NoSuchElementException exception at you. Sigh.

To avoid it insert integers as NumberInts

db.person.insert({firstName: "Eugene", lastName: "Platonov", age: NumberInt(27)})  

I think reactive mongo should be more explicit about what went wrong and hopefully this will be fixed soon, but if not, you know where the dog lies buried.

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