build.sbt

//Works
mergeStrategy in assembly <<= (mergeStrategy in assembly) { (old) =>
  {
    case x if "clever/common/.*".r.pattern.matcher(x).matches =>
      MergeStrategy.discard
    case x if "clever/db/.*".r.pattern.matcher(x).matches =>
      MergeStrategy.discard
    case x => old(x)
  }
}

Build.scala
//Do not work
  lazy val manager = Project(id = "manager", base = file("manager"))
    .dependsOn(common, db)
    .aggregate(common, db)
    .settings(
      mergeStrategy in assembly <<= (mergeStrategy in assembly) { (old) =>
        {
          case x if "clever/common/.*".r.pattern.matcher(x).matches =>
            MergeStrategy.discard
          case x if "clever/db/.*".r.pattern.matcher(x).matches =>
            MergeStrategy.discard
          case x => old(x)
        }
      }
    )