What is Slick and why we use it?Mark van der Tol

Why we moved to SQLMongoDB has• No joins or referential integrity• No transactions• Max element size for indexAdvantages PostgreSQL has• Better indexes• Better tooling• Strict schema

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Problem faced with MongoDBQuery unseen alerts for usersData:{ "message": "Example alert", "seenBy": [...]}Query:{ "seenBy": { "$ne": "user" }}Max element size for indexed elements: 1 KB

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Trivial in SQLSELECT messageFROM alertsWHERE id NOT IN ( SELECT messageId FROM views WHERE user = "user")

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MongoDB• We keep using Mongo for statistics data• Easier to shard/replicate• No schema enforcement

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What is Slick• Database query library for Scala

• Table mapping• Strongly typed• Collection like syntax

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Available for • DB2*• Derby/JavaDB• H2• HSQLDB/HyperSQL• Microsoft Access• Microsoft SQL Server• MySQL• Oracle*• PostgreSQL• SQLite*Requires subscription for production use

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

Database.forURL("jdbc:postgresql://…", driver = "org.postgresql.Driver") withSession { //session is now implicitly available in thread-local storage}

Database.forURL("jdbc:postgresql://…", driver = "org.postgresql.Driver") withTransaction { //session is now implicitly available in thread-local storage}

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Table definition

object CoffeeTable extends Table[(String, BigDecimal, Int)]("COFFEE") {

def name = column[String]("NAME", O.PrimaryKey) def price = column[BigDecimal]("PRICE") def sales = column[Int]("SALES")

def * = name ~ price ~ sales} 9

Simple SELECT queryval minPrice: BigDecimal = 1.0

val query = for { c <- CoffeeTable if (c.price >= minPrice)} yield (c.name)

val names = query.list10

Table definition for Case classcase class Coffee (name: String, price: BigDecimal, sales: Int)

object CoffeeTable extends Table[Coffee]("coffee") {

def name = column[String]("NAME", O.PrimaryKey) def price = column[BigDecimal]("PRICE") def sales = column[Int]("SALES") def * = name ~ price ~ sales <> (Coffee, Coffee.unapply _)} 11

Simple SELECT query

val query = for { c <- CoffeeTable} yield (c)

val coffees = query.list

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SELECT query with join

val query = for { p <- PersonTable c <- CoffeeTable if (p.favoriteCoffee === c.name)} yield (p.name, c.name, c.price)

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INSERT query

CoffeeTable.insert( Coffee("Java", 2.50, 0))

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UPDATE/DELETE queryval query = for { c <- CoffeeTable if (c.price < 1.50)} yield (c.price)

query.update(1.50)

query.delete15

Plain SQL queries

val query = StaticQuery .query[BigDecimal, (String, Int)]( """select c.name, c.sales from coffees c where c.price < ?""")

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Features not shown

• Queries with parameters• Extensible: Add own types and functions• "Direct embedding"

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Features

• Query library that stays close to SQL• Hides dbms specific syntax• Prevents SQL-injections• Many checks compile time

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Disadvantages

• DSL not always intuitive• Difficult compile errors• Focus on single thread usage• Not very comprehensive documentation

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Multi-threaded use workaround val session = Database.forURL("jdbc:…", driver = “…").createSession()

session.conn.setAutoCommit(false)//prevent Slick from creating transactions itselfsession.asInstanceOf[BaseSession].inTransaction = true

…

session.conn.commit()session.close()

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Resources

• Slick:http://slick.typesafe.com/http://groups.google.com/group/scalaquery

• Sheets:http://www.plotprojects.com/

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THANK YOU 22