scala slick method I can not understand so far
I try to understand some Slick works and what it requires.
Here it an example:
package models
case class Bar(id: Option[Int] = None, name: String)
object Bars extends Table[Bar]("bar") {
def id = column[Int]("id", O.PrimaryKey, O.AutoInc)
// This is the primary key column
def name = column[String]("name")
// Every table needs a * projection with the same type as the table's type parameter
def * = id.? ~ name <>(Bar, Bar.unapply _)
}
Could somebody explain me what's the purpose of * method here, what is <>, why unapply? and what is Projection - method ~' returns the instance of Projection2?
Answer
[UPDATE] - added (yet another) explanation on for comprehensions
The
*method:This returns the default projection - which is how you describe:
'all the columns (or computed values) I am usually interested' in.
Your table could have several fields; you only need a subset for your default projection. The default projection must match the type parameters of the table.
Let's take it one at a time. Without the
<>stuff, just the*:// First take: Only the Table Defintion, no case class: object Bars extends Table[(Int, String)]("bar") { def id = column[Int]("id", O.PrimaryKey, O.AutoInc) def name = column[String]("name") def * = id ~ name // Note: Just a simple projection, not using .? etc } // Note that the case class 'Bar' is not to be found. This is // an example without it (with only the table definition)Just a table definition like that will let you make queries like:
implicit val session: Session = // ... a db session obtained from somewhere // A simple select-all: val result = Query(Bars).list // result is a List[(Int, String)]the default projection of
(Int, String)leads to aList[(Int, String)]for simple queries such as these.// SELECT b.name, 1 FROM bars b WHERE b.id = 42; val q = for (b <- Bars if b.id === 42) yield (b.name ~ 1) // yield (b.name, 1) // this is also allowed: // tuples are lifted to the equivalent projection.What's the type of
q? It is aQuerywith the projection(String, Int). When invoked, it returns aListof(String, Int)tuples as per the projection.val result: List[(String, Int)] = q.listIn this case, you have defined the projection you want in the
yieldclause of theforcomprehension.Now about
<>andBar.unapply.This provides what are called Mapped Projections.
So far we've seen how slick allows you to express queries in Scala that return a projection of columns (or computed values); So when executing these queries you have to think of the result row of a query as a Scala tuple. The type of the tuple will match the Projection that is defined (by your
forcomprehension as in the previous example, of by the default*projection). This is whyfield1 ~ field2returns a projection ofProjection2[A, B]whereAis the type offield1andBis the type offield2.q.list.map { case (name, n) => // do something with name:String and n:Int } Queury(Bars).list.map { case (id, name) => // do something with id:Int and name:String }We're dealing with tuples, which may be cumbersome if we have too many columns. We'd like to think of results not as
TupleNbut rather some object with named fields.(id ~ name) // A projection // Assuming you have a Bar case class: case class Bar(id: Int, name: String) // For now, using a plain Int instead // of Option[Int] - for simplicity (id ~ name <> (Bar, Bar.unapply _)) // A MAPPED projection // Which lets you do: Query(Bars).list.map ( b.name ) // instead of // Query(Bars).list.map { case (_, name) => name } // Note that I use list.map instead of mapResult just for explanation's sake.How does this work?
<>takes a projectionProjection2[Int, String]and returns a mapped projection on the typeBar. The two argumentsBar, Bar.unapply _tell slick how this(Int, String)projection must be mapped to a case class.This is a two-way mapping;
Baris the case class constructor, so that's the information needed to go from(id: Int, name: String)to aBar. Andunapplyif you've guessed it, is for the reverse.Where does
unapplycome from? This is a standard Scala method available for any ordinary case class - just definingBargives you aBar.unapplywhich is an extractor that can be used to get back theidandnamethat theBarwas built with:val bar1 = Bar(1, "one") // later val Bar(id, name) = bar1 // id will be an Int bound to 1, // name a String bound to "one" // Or in pattern matching val bars: List[Bar] = // gotten from somewhere val barNames = bars.map { case Bar(_, name) => name } val x = Bar.unapply(bar1) // x is an Option[(String, Int)]So your default projection can be mapped to the case class you most expect to use:
object Bars extends Table[Bar]("bar") { def id = column[Int]("id", O.PrimaryKey, O.AutoInc) def name = column[String]("name") def * = id ~ name <>(Bar, Bar.unapply _) }Or you can even have it per-query:
case class Baz(name: String, num: Int) // SELECT b.name, 1 FROM bars b WHERE b.id = 42; val q1 = for (b <- Bars if b.id === 42) yield (b.name ~ 1 <> (Baz, Baz.unapply _))Here the type of
q1is aQuerywith a mapped projection toBaz. When invoked, it returns aListofBazobjects:val result: List[Baz] = q1.listFinally, as an aside, the
.?offers Option Lifting - the Scala way of dealing with values that may not be.(id ~ name) // Projection2[Int, String] // this is just for illustration (id.? ~ name) // Projection2[Option[Int], String]Which, wrapping up, will work nicely with your original definition of
Bar:case class Bar(id: Option[Int] = None, name: String) // SELECT b.id, b.name FROM bars b WHERE b.id = 42; val q0 = for (b <- Bars if b.id === 42) yield (b.id.? ~ b.name <> (Bar, Bar.unapply _)) q0.list // returns a List[Bar]In response to the comment on how Slick uses
forcomprehensions:Somehow, monads always manage to show up and demand to be part of the explanation...
For comprehensions are not specific to collections only. They may be used on any kind of Monad, and collections are just one of the many kinds of monad types available in Scala.
But as collections are familiar, they make a good starting point for an explanation:
val ns = 1 to 100 toList; // Lists for familiarity val result = for { i <- ns if i*i % 2 == 0 } yield (i*i) // result is a List[Int], List(4, 16, 36, ...)In Scala, a for comprehension is syntactic sugar for method (possibly nested) method calls: The above code is (more or less) equivalent to:
ns.filter(i => i*i % 2 == 0).map(i => i*i)Basically, anything with
filter,map,flatMapmethods (in other words, a Monad) can be used in aforcomprehension in place ofns. A good example is the Option monad. Here's the previous example where the sameforstatement works on both theListas well asOptionmonads:// (1) val result = for { i <- ns // ns is a List monad i2 <- Some(i*i) // Some(i*i) is Option if i2 % 2 == 0 // filter } yield i2 // Slightly more contrived example: def evenSqr(n: Int) = { // return the square of a number val sqr = n*n // only when the square is even if (sqr % 2 == 0) Some (sqr) else None } // (2) result = for { i <- ns i2 <- evenSqr(i) // i2 may/maynot be defined for i! } yield i2In the last example, the transformation would perhaps look like this:
// 1st example val result = ns.flatMap(i => Some(i*i)).filter(i2 => i2 %2 ==0) // Or for the 2nd example result = ns.flatMap(i => evenSqr(i))In Slick, queries are monadic - they are just objects with the
map,flatMapandfiltermethods. So theforcomprehension (shown in the explanation of the*method) just translates to:val q = Query(Bars).filter(b => b.id === 42).map(b => b.name ~ 1) // Type of q is Query[(String, Int)] val r: List[(String, Int)] = q.list // Actually run the queryAs you can see,
flatMap,mapandfilterare used to generate aQueryby the repeated transformation ofQuery(Bars)with each invocation offilterandmap. In the case of collections these methods actually iterate and filter the collection but in Slick they are used to generate SQL. More details here: How does Scala Slick translate Scala code into JDBC?