cf. https://github.com/scalikejdbc/scalikejdbc/blob/master/scalikejdbc-mapper-generator/src/main/scala/scalikejdbc/mapper/SbtPlugin.scala

scalikejdbc_build2.sbt

lazy val codeGenTask2 = TaskKey[Unit]("hoge", "description")
codeGenTask2 := {
  import scalikejdbc.mapper.SbtKeys._
  scalikejdbcCodeGeneratorAll.in(Compile).value.apply(scalikejdbcJDBCSettings.in(Compile).value, scalikejdbcGeneratorSettings.in(Compile).value).foreach { g =>
    g.writeModel()
    // g.writeSpec(g.specAll())
  }
}
compile in Compile <<= (compile in Compile).dependsOn(codeGenTask2)