Architecturally Elegant - lahteenmaki.net

Querying with Scala

Jyri-Matti Lähteenmäki

2011-02-14

Tags: orm jpa2 scala scala sql sql

Let's say we have a simple domain model with departments and employees (behold my imagination...). Forget all persistence or SQL related stuff, let's just have it all in-memory:

object InMemory {
  case class Employee(name: String, salary: Option[Int])
  case class Department(name: String, employees: Set[Employee])

  val jack = Employee("Jack Janitor", Some(2000))
  val jill = Employee("Jill Jitter", None)
  val matt = Employee("Matt Manager", Some(3250))
  val sarah = Employee("Sarah Surrender", Some(3000))
  val bill = Employee("Bill Biller", Some(4500))

  def employees = Seq(jack, jill, matt, sarah, bill)
  def departments = Seq(Department("Research and Development", Set(bill, sarah)),
                        Department("IT", Set(jack, jill)),
                        Department("Management", Set(matt)))
}

How about querying the data?

Since the language of this example is Scala, I would like to write the queries in Scala. Had I implemented this in Java, I would be wanting to walk the object graph iterating collections. But I cannot just go and iterate through all the employees of a department to find those whose salary is high enough, since in real life that might cause all the employees of the department to be loaded from the database, in some cases one-by-one. So I'm forced to use some silly JPQL or a criteria query to give the system the power to properly optimize my actions. The important thing here is that what I really want to do is not to iterate through a collection, but to declare that I'm interested in employees belonging to a certain department. The iteration is just the implementation of this problem in Java. As a friend of mine said, I'm over-specifying the problem by performing the iteration.

Scala does not force this over-specification. I can use for-comprehension for querying, which is quite abstract regarding what's actually happening behind the scenes:

import InMemory._

val wellPaidEmployees = for {
  d <- departments
  e <- d.employees if e.salary.isDefined && e.salary.get > 3000
} yield e

val namesAndSalariesOfRnDEmployees = for {
  d <- departments if d.name startsWith "Research"
  e <- d.employees
} yield (e.name, e.salary)

val underpaidEmployees = for {
  e <- employees if !e.salary.isDefined || e.salary.get + 100 < 3300
} yield (e.name, e.salary.getOrElse(0) % 42)

The syntax Scala offers is actually so abstract, that it shows in no way that I'm actually picking stuff from in-memory collections. This immediately raises an interesting question: what exactly is needed to move this data to an SQL database?

First of all, the case classes defining the model are a bit too in-memory-specific. Let's change them a bit:

import engine._
import engine.Types._
import engine.Scalaq._
package External {
  class Department extends Table {
    val name = $[String]
    val employees: ->*[Employee] = ->*(_.department)
  }
  class Employee extends Table {
    val name = $[String]
    val salary = ?[Int]
    val department: ->[Department] = ->(_.employees)
  }
  def departments = new Department
  def employees = new Employee
}

This is actually declaring the same information, but it also builds a model of the model, i.e. a meta model. Forgive my choice of "names" to define properties and relations, I have a bad habit to sometimes strip away unnecessary characters =). Now by changing the import of InMemory to External in the query examples, the same code compiles. This is exactly what I want. The type of the data storage should not affect my queries, since I'm not querying the database, I'm querying the data.

At this point you might be thinking: Hey, this idiot is trying to build yet another tool to abstract away SQL completely from the application. That's not my intention at all. Abstraction is always a compromise. When we abstract away the fact that our data store is an SQL database, we give away a bunch of tools it provides. There are and always will be queries so complex or so resource-hungry that one just cannot give a satisfying implementation without assuming an SQL backend. At some point that's not enough, and one needs to know it's an Oracle 11g database. Therefore, every abstraction like this should only strive to solve 95% or so of the cases.

Back to the queries. After changing the import clause the for-comprehensions don't return the actual data anymore, they return some objects containing the information needed to later construct the actual query against the data store, whatever it is. You might have noticed that none of the example codes had anything related to SQL (well, the base class name `Table should probably be something else...). If we add some jdbc-connection-related helper methods (not listed), we can actually perform these queries against a database:

val Seq(a,b,c) = transaction("jdbc:h2:mem:test") { implicit c =>
  import engine.sql._
  val session = new Session with H2Dialect
  import session._
  execute(generateDDL(departments, employees))
  testData foreach execute
  Seq(executeQuery(generateQuery(wellPaidEmployees)),
      executeQuery(generateQuery(namesAndSalariesOfRnDEmployees)),
      executeQuery(generateQuery(underpaidEmployees)))
}

First the SQL schema is created based on the model definition and populated with some test data. Then the SQL corresponding to the queries are generated and the resulting strings executed. Printing the final three string objects will print the actual results of the queries.

The current implementation of the engine is rather simple with a few hundred lines of somewhat readable Scala. This means that although implicits are being used quite heavily, the concept as a whole is still quite easily comprehensible.

So, is this somehow revolutionary? Hell no. It's a simple example performing simple queries. All the important stuff like composability or alternative data stores are still missing. On the other hand, does e.g. JPA have those properties?

Various nice features can be spotted in this implementation (or could be, if you looked at my source code):

Aggregate functions seem also implementable, though I don't yet have them finished. Composition is something I must experiment with soon since it's an important feature. Other experiments include inserts/updates, populating objects with the data easily, some other data store types... These might bring some additional noise to the model declaration but hopefully keep the queries abstract.

There is a project called ScalaQuery which has implemented something like this. I do not like it's approach, though, which is stated in its overview in the web site (I highlighted the annoying parts):

ScalaQuery is an API / DSL (domain specific language) built on top of JDBC for accessing relational databases in Scala.

I consider basic querying as an abstract thing having no relation to the type of the data store, but ScalaQuery is making ties to things like JDBC and SQL. This is also visible in its syntax. I haven't yet found a need to make that kind of deviations from regular Scala, but it might be that I just haven't been there yet.

The examples I've given are just my initial experiments, and the syntax is most likely going to change at least slightly. I'm hoping that additional features won't force me to bring any additional verbosity, though. I will post a working jar-file later so that you can try it if you have any interest. I will also post all source code in the future, when I'm done enough experimenting.

If You have any thoughts of this kind of abstract querying in Scala, I'd be glad to hear your thoughts. Now I'm heading to JFokus, see you there.