Invokers

Invokers are the components of a Slick query that build up the SQL select statement. Slick exposes a variety of invokers that allow the construction of complex queries. Let's look at some of these invokers here:

  • The map invoker is useful to select individual columns or apply operations to columns:
    scala> db.withSession { implicit session =>
  Tables.transactions.map {
    _.candidate 
  }.take(5).list       
}
List[String] = List(Obama, Barack, Paul, Ron, Paul, Ron, Paul, Ron, Obama, Barack)
  • The filter invoker is the equivalent of the WHERE statements in SQL. Note that Slick fields must be compared using ===:
    scala> db.withSession { implicit session => 
  Tables.transactions.filter {
    _.candidate === "Obama, Barack"
  }.take(5).list
}
List[Tables.Transactions#TableElementType] = List(Transaction(Some(1),Obama, Barack,Doe, John,TX,None,200,2010-06-22), ...

    Similarly, to filter out donations to Barack Obama, use the =!= operator:

    scala> db.withSession { implicit session =>   
  Tables.transactions.filter { 
    _.candidate =!= "Obama, Barack"
  }.take(5).list
}
List[Tables.Transactions#TableElementType] = List(Transaction(Some(2),Paul, Ron,BROWN, TODD W MR.,OH,...
  • The sortBy invoker is the equivalent of the ORDER BY statement in SQL:
    scala> db.withSession { implicit session =>     
  Tables.transactions.sortBy { 
    _.date.desc 
  }.take(5).list
}
List[Tables.Transactions#TableElementType] = List(Transaction(Some(65536),Obama, Barack,COPELAND, THOMAS,OH,Some(COLLEGE TEACHING),10000,2012-01-02)
  • The leftJoin, rightJoin, innerJoin, and outerJoin invokers are used for joining tables. As we do not cover interactions between multiple tables in this tutorial, we cannot demonstrate joins. See the Slick documentation (http://slick.typesafe.com/doc/2.1.0/queries.html#joining-and-zipping) for examples of these.
  • Aggregation invokers such as length, min, max, sum, and avg can be used for computing summary statistics. These must be executed using .run, rather than .list, as they return single numbers. For instance, to get the total donations to Barack Obama:
    scala> db.withSession { implicit session => 
  Tables.transactions.filter {
    _.candidate === "Obama, Barack"
  }.map { _.amount  }.sum.run
}
Option[Int] = Some(849636799) // (in cents)
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