As soon as you construct a MongoClient object, a few type conversions will be loaded automatically for you - Scala’s built-in regular expressions (e.g. "\\d{4}-\\d{2}-\\d{2}".r will now serialize to MongoDB automatically with no work from you), as well as a few other things. The general idea is that common Java types (such as ArrayList) will be returned as the equivalent Scala type.

If you find you need to unload the default helpers, you can load and unload them easily:

import com.mongodb.casbah.commons.conversions.scala._

Joda Time

Many Scala developers tend to prefer Joda time over JDK Dates, you can also explicitly enable serialization and deserialization of them (w/ full support for the Scala-Time wrappers) by an explicit call:

import com.mongodb.casbah.commons.conversions.scala._

Once these are loaded, Joda Time (and Scala Time wrappers) will be saved to MongoDB as proper BSON Dates, and on retrieval / deserialisation all BSON Dates will be returned as Joda DateTime instead of a JDK Date (aka java.util.Date). Because this can cause problems in some instances, you can explicitly unload the Joda Time helpers:

import com.mongodb.casbah.commons.conversions.scala._

If you prefer Joda LocalDateTime - theres also a conversion helper for that:

import com.mongodb.casbah.commons.conversions.scala._

// Remove the helper

Custom Conversion Helpers

Writing your own conversion helper is relatively easy, simply provide a BSON.addEncodingHook(encodeType, transformer) and a BSON.removeEncodingHooks(encodeType).

See the casbah conversions for an example of creating your own.