How to write database-agnostic Play application an

Question

I'm using Slick with a Play Framework 2.1 and I have some troubles.

Given the following entity...

package models

import scala.slick.driver.PostgresDriver.simple._

case class Account(id: Option[Long], email: String, password: String)

object Accounts extends Table[Account]("account") {
  def id = column[Long]("id", O.PrimaryKey, O.AutoInc)
  def email = column[String]("email")
  def password = column[String]("password")
  def * = id.? ~ email ~ password <> (Account, Account.unapply _)
}

...I have to import a package for a specific database driver, but I want to use H2 for testing and PostgreSQL in production. How should I proceed?

I was able to workaround this by overriding the driver settings in my unit test:

package test

import org.specs2.mutable._

import play.api.test._
import play.api.test.Helpers._

import scala.slick.driver.H2Driver.simple._
import Database.threadLocalSession

import models.{Accounts, Account}

class AccountSpec extends Specification {

  "An Account" should {
    "be creatable" in {
      Database.forURL("jdbc:h2:mem:test1", driver = "org.h2.Driver") withSession {
        Accounts.ddl.create                                                                                                                                          
        Accounts.insert(Account(None, "[e-mail]", "Password"))
        val account = for (account <- Accounts) yield account
        account.first.id.get mustEqual 1
      }
    }
  }
}

I don't like this solution and I'm wondering if there is an elegant way to write DB-agnostic code so there are two different database engines used - one in testing and another in production?

I don't want to use evolution, either, and prefer to let Slick create the database tables for me:

import play.api.Application
import play.api.GlobalSettings
import play.api.Play.current
import play.api.db.DB

import scala.slick.driver.PostgresDriver.simple._
import Database.threadLocalSession

import models.Accounts

object Global extends GlobalSettings {

  override def onStart(app: Application) {
    lazy val database = Database.forDataSource(DB.getDataSource())

    database withSession {
      Accounts.ddl.create
    }
  }
}

The first time I start the application, everything works fine... then, of course, the second time I start the application it crashes because the tables already exist in the PostgreSQL database.

That said, my last two questions are:

1. How can I determine whether or not the database tables already exist?
2. How can I make the onStart method above DB-agnostic so that I can test my application with FakeApplication?

Answer 1

If, like me, you're not using Play! for the project, a solution is provided by Nishruu here

Answer 2

The play-slick does exactly the same as what is proposed in the other answers, and it seems to be under the umbrella of Play/Typesafe.

You just can import import play.api.db.slick.Config.driver.simple._ and it will choose the appropriate driver according to conf/application.conf.

It also offers some more things like connection pooling, DDL generation...

Answer 3

You find an example on how to use the cake pattern / dependency injection to decouple the Slick driver from the database access layer here: https://github.com/slick/slick-examples.

How to decouple the Slick driver and test application with FakeApplication

A few days ago I wrote a Slick integration library for play, which moves the driver dependency to the application.conf of the Play project: https://github.com/danieldietrich/slick-integration.

With the help of this library your example would be implemented as follows:

1) Add the dependency to project/Build.scala

"net.danieldietrich" %% "slick-integration" % "1.0-SNAPSHOT"

Add snapshot repository

resolvers += "Daniel's Repository" at "http://danieldietrich.net/repository/snapshots"

Or local repository, if slick-integration is published locally

resolvers += Resolver.mavenLocal

2) Add the Slick driver to conf/application.conf

slick.default.driver=scala.slick.driver.H2Driver

3) Implement app/models/Account.scala

In the case of slick-integration, it is assumed that you use primary keys of type Long which are auto incremented. The pk name is 'id'. The Table/Mapper implementation has default methods (delete, findAll, findById, insert, update). Your entities have to implement 'withId' which is needed by the 'insert' method.

package models

import scala.slick.integration._

case class Account(id: Option[Long], email: String, password: String)
    extends Entity[Account] {
  // currently needed by Mapper.create to set the auto generated id
  def withId(id: Long): Account = copy(id = Some(id))
}

// use cake pattern to 'inject' the Slick driver
trait AccountComponent extends _Component { self: Profile =>

  import profile.simple._

  object Accounts extends Mapper[Account]("account") {
    // def id is defined in Mapper
    def email = column[String]("email")
    def password = column[String]("password")
    def * = id.? ~ email ~ password <> (Account, Account.unapply _)
  }

}

4) Implement app/models/DAL.scala

This is the Data Access Layer (DAL) which is used by the controllers to access the database. Transactions are handled by the Table/Mapper implementation within the corresponding Component.

package models

import scala.slick.integration.PlayProfile
import scala.slick.integration._DAL
import scala.slick.lifted.DDL

import play.api.Play.current

class DAL(dbName: String) extends _DAL with AccountComponent
    /* with FooBarBazComponent */ with PlayProfile {

  // trait Profile implementation
  val profile = loadProfile(dbName)
  def db = dbProvider(dbName)

  // _DAL.ddl implementation
  lazy val ddl: DDL = Accounts.ddl // ++ FooBarBazs.ddl

}

object DAL extends DAL("default")

5) Implement test/test/AccountSpec.scala

package test

import models._
import models.DAL._
import org.specs2.mutable.Specification
import play.api.test.FakeApplication
import play.api.test.Helpers._
import scala.slick.session.Session

class AccountSpec extends Specification {

  def fakeApp[T](block: => T): T =
    running(FakeApplication(additionalConfiguration = inMemoryDatabase() ++
        Map("slick.default.driver" -> "scala.slick.driver.H2Driver",
          "evolutionplugin" -> "disabled"))) {
      try {
        db.withSession { implicit s: Session =>
          try {
            create
            block
          } finally {
            drop
          }
        }
      }
    }

  "An Account" should {
    "be creatable" in fakeApp {
      val account = Accounts.insert(Account(None, "[e-mail]", "Password"))
      val id = account.id
      id mustNotEqual None 
      Accounts.findById(id.get) mustEqual Some(account)
    }
  }

}

How to determine whether or not the database tables already exist

I cannot give you a sufficient answer to this question...

... but perhaps this is not really s.th you want to do. What if you add an attribute to an table, say Account.active? If you want to safe the data currently stored within your tables, then an alter script would do the job. Currently, such an alter script has to be written by hand. The DAL.ddl.createStatements could be used to retrieve the create statements. They should be sorted to be better comparable with previous versions. Then a diff (with previous version) is used to manually create the alter script. Here, evolutions are used to alter the db schema.

Here's an example on how to generate (the first) evolution:

object EvolutionGenerator extends App {

  import models.DAL

  import play.api.test._
  import play.api.test.Helpers._

    running(FakeApplication(additionalConfiguration = inMemoryDatabase() ++
        Map("slick.default.driver" -> "scala.slick.driver.PostgresDriver",
          "evolutionplugin" -> "disabled"))) {


    val evolution = (
      """|# --- !Ups
         |""" + DAL.ddl.createStatements.mkString("\n", ";\n\n", ";\n") +
      """|
         |# --- !Downs
         |""" + DAL.ddl.dropStatements.mkString("\n", ";\n\n", ";\n")).stripMargin

    println(evolution)

  }

}

Answer 4

I was also trying to address this problem: the ability to switch databases between test and production. The idea of wrapping each table object in a trait was unappealing.

I am not trying to discuss the pros and cons of the cake pattern here, but I found another solution, for those who are interested.

Basically, make an object like this:

package mypackage
import scala.slick.driver.H2Driver
import scala.slick.driver.ExtendedProfile
import scala.slick.driver.PostgresDriver

object MovableDriver {
  val simple = profile.simple
  lazy val profile: ExtendedProfile = {
    sys.env.get("database") match {
      case Some("postgres") => PostgresDriver
      case _ => H2Driver
    }
  }
}

Obviously, you can do any decision logic you like here. It does not have to be based on system properties.

Now, instead of:

import scala.slick.driver.H2Driver.simple._

You can say

import mypackage.MovableDriver.simple._

UPDATE: A Slick 3.0 Version, courtesy of trent-ahrens:

package mypackage

import com.typesafe.config.ConfigFactory

import scala.slick.driver.{H2Driver, JdbcDriver, MySQLDriver}

object AgnosticDriver {
  val simple = profile.simple
  lazy val profile: JdbcDriver = {
    sys.env.get("DB_ENVIRONMENT") match {
      case Some(e) => ConfigFactory.load().getString(s"$e.slickDriver") match {
        case "scala.slick.driver.H2Driver" => H2Driver
        case "scala.slick.driver.MySQLDriver" => MySQLDriver
      }
      case _ => H2Driver
    }
  }
}