Liz Douglass

Migrating to Scala 2.8

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A few months ago Scala 2.8.0 was released. We migrated our project from version 2.7.7 quite soon after the announcement. Making the switch was quite straight forward, in fact all we needed to do was change the build.scala.versions property in our Simple Build Tool build.properties file. Admittedly it took several hours to fix all the compilation errors, but once this was done we found we found we were able to make our code base more readable because of two shiny new things in particular:

1. Collections:

As I’ve written about before, we are using MongoDB for persistence. We are also using the MongoDB Java driver. This library makes heavy use of BasicDBObjects. These are the simplest implementation of the DBObject interface. This interface represents “A key-value map that can be saved to the database”. BasicDBObjects are used for more than just inserting data. In fact they are also used extensively in many of the methods defined on the DBCollection class. For example, this is the definition of the update method:

WriteResult update(DBObject q, DBObject o)

(Source: class com.mongodb.DBCollection)
(q is the query used to find the correct element and o is the update operation that should be performed)

We often use the BasicDBObject constructor that takes a Java map. When were using Scala 2.7.7 we had a library to handle the conversion of maps from Scala to Java. We ended up with a lot of repository methods that were littered with calls to asJava, like the one below:

import org.scala_tools.javautils.Imports._
import com.mongodb._
import org.bson.types.ObjectId

def updateLastLogin(personId: String) {
   val criteria = new BasicDBObject(Map("_id" -> new ObjectId(personId)).asJava)
   val action = new BasicDBObject(Map("$set" -> new BasicDBObject(Map("lastLogin" -> new Date).asJava)).asJava)
   collection.update(criteria, action)
}

Now in Scala 2.8 there is a new mechanism for converting collection types from Scala to Java and thankfully all the asJava calls have disappeared from our codebase. Now we have cleaner functions like this one:

def markLogin(memberId: String) = set(new MemberId(memberId).asObjectId, "lastLogin" -> new Date)

Where:

def set(id: ObjectId, keyValues: (String, Any)*) = collection.update(Map("_id" -> id), $set (keyValues: _*), false, false)

Note that we are now also using the Casbah library. The update method above is defined on the MongoCollectionWrapper in the Casbah library.

2. JSON

We have a Scala backend API and a Django frontend. The backend serves JSON to our frontend. We are using the Lift Json library to do both the serialising in the backend webapp, as well as the de-serialising in our integration tests. Some of our tests use the Lift Json Library “LINQ” style JSON parsing, while others use the “Xpath” style (both of which are described here):

LINQ style:

import net.liftweb.json.JsonParser._
import net.liftweb.json.JsonAST._
...

val (data, status) = ApiClient.get(myUri)
val json = parse(data)

val myValues = for{JField("something", JString(myValue)) <- (json \ "somethingElse")} yield myValue
myValues should have length (3)
myValues should (
    contain("frog")
    and contain("tennis")
    and contain("phone")
)

Xpath style:

scenario("succesfully view something") {
   val json = getMemberLiftJson(someId)
   assert((json \ "bar").extract[String] === "ExpectedString")
}

The Xpath style is readable and compact and we’ve retained quite a few of these type of tests in our code base. Personally I find the LINQ style quite confusing. I need to refer to the Lift project github site every time I try to use it. In Scala 2.8 there is now JSON support and we have instead been using this for parsing in the tests. Specifically, we have been using the parseFull method like so:

scenario("bar") {
    val (content, status) = ApiClient.get(someUri)
    JSON.parseFull(content) match {
        case Some(data : List[Any]) => {
            data.length should be(4)
            data should not contain("Internet")
        }
        case json => fail("Couldn't find a list in response %s".format(json))
    }
}

Go Scala 2.8!

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Written by lizdouglass

November 1, 2010 at 9:48 pm

Posted in Uncategorized

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