Add possibility to define bidirectional migrations

stamina-core/src/main/scala/stamina/migrations.scala

@@ -11,22 +11,12 @@ package object migrations {
   /** The identity Migration will always return its input as its output. */
   def identityMigration[T]: Migration[T] = identity[T]
 
-  /**
-   * Adds support for combining two instances of Migration[T] into a new
-   * Migration[T] that will apply the first one and then the second one.
-   */
-  implicit class MigrationWithComposition[T](val firstMigration: Migration[T]) extends AnyVal {
-    def &&(secondMigration: Migration[T]): Migration[T] = {
-      (value: T) ⇒ secondMigration(firstMigration(value))
-    }
-  }
-
   /**
    * Creates a Migrator[T, V1] that can function as a builder for
    * creating Migrator[T, V2], etc. Its migration will be the identity
    * function so calling its migrate function will not have any effect.
    */
-  def from[T, V <: V1: VersionInfo]: Migrator[T, V] = new Migrator[T, V](Map(Version.numberFor[V] → identityMigration[T]))
+  def from[T, V <: V1: VersionInfo]: Migrator[T, V] = new Migrator[T, V](Map(Version.numberFor[V] → identityMigration[T]), Map.empty)
 }
 
 package migrations {
@@ -35,8 +25,8 @@ package migrations {
     extends RuntimeException(s"No migration defined from version ${fromVersion} to version ${toVersion}.")
 
   /**
-   * A `Migrator[R, V]` can migrate raw values of type R from older
-   * versions to version `V` by applying a specific `Migration[R]` to it.
+   * A `Migrator[R, V]` can migrate raw values of type R between
+   * two versions by applying all `Migration[R]`s between these versions.
    *
    * You can create instances of `Migrator[R, V]` by using
    * a small type-safe DSL consisting of two parts: the
@@ -55,20 +45,36 @@ package migrations {
    * }}}
    *
    *  @tparam R The type of raw data being migrated. In the JSON implementation this would be `JsValue`.
-   *  @tparam V The "current" version of this Migrator, i.e. it can migrate values from V1 to this version or any version in between.
+   *  @tparam V The "current" version of this Migrator.
    */
-  class Migrator[R, V <: Version: VersionInfo] private[stamina] (migrations: Map[Int, Migration[R]] = Map.empty) {
-    def canMigrate(fromVersion: Int): Boolean = migrations.contains(fromVersion)
+  class Migrator[R, V <: Version: VersionInfo] private[stamina] (forwardMigrations: Map[Int, Migration[R]] = Map.empty, backwardMigrations: Map[Int, Migration[R]] = Map.empty) {
+    def canMigrate(fromVersion: Int, toVersion: Int): Boolean = forwardMigrations.contains(fromVersion) && forwardMigrations.contains(toVersion)
 
-    def migrate(value: R, fromVersion: Int): R = {
-      migrations.get(fromVersion).map(_.apply(value)).getOrElse(
-        throw UndefinedMigrationException(fromVersion, Version.numberFor[V])
-      )
+    def migrate(value: R, fromVersion: Int, toVersion: Int): R = {
+      def applyMigrationOn(value: R, migration: Migration[R]) = migration(value)
+
+      if (fromVersion <= toVersion) {
+        (fromVersion to toVersion).drop(1)
+          .map(version ⇒ forwardMigrations.getOrElse(version, throw UndefinedMigrationException(fromVersion, toVersion)))
+          .foldLeft(value)(applyMigrationOn)
+      } else {
+        (toVersion to fromVersion).drop(1).reverse
+          .flatMap(backwardMigrations.get)
+          .foldLeft(value)(applyMigrationOn)
+      }
     }
 
     def to[NextV <: Version: VersionInfo](migration: Migration[R])(implicit isNextAfter: IsNextVersionAfter[NextV, V]) = {
       new Migrator[R, NextV](
-        migrations.mapValues(_ && migration) + (Version.numberFor[NextV] → identityMigration[R])
+        forwardMigrations + (Version.numberFor[NextV] → migration),
+        backwardMigrations
+      )
+    }
+
+    def backTo[PrevV <: Version: VersionInfo](migration: Migration[R])(implicit e: IsNextVersionAfter[V, PrevV]) = {
+      new Migrator[R, V](
+        forwardMigrations,
+        backwardMigrations + (Version.numberFor[V] → migration)
       )
     }
   }

stamina-core/src/test/scala/stamina/migrations/MigratorSpec.scala

@@ -0,0 +1,85 @@
+package stamina.migrations
+
+import stamina._
+
+class MigratorSpec extends StaminaSpec {
+
+  val migrator = from[String, V1]
+    .to[V2](_ + "V2")
+    .backTo[V1](_.replace("V2", ""))
+    .to[V3](_ + "V3")
+    .backTo[V2](_.replace("V3", ""))
+    .to[V4](_ + "V4")
+    .backTo[V3](_.replace("V4", ""))
+    .to[V5](_ + "V5")
+
+  "Migrator V5" should {
+    "be able to migrate" when {
+      "migration is from V1 to V5" in {
+        migrator.canMigrate(1, 5) shouldBe true
+      }
+
+      "migration is from V1 to V1" in {
+        migrator.canMigrate(1, 1) shouldBe true
+      }
+
+      "migration is from V5 to V2" in {
+        migrator.canMigrate(5, 2) shouldBe true
+      }
+    }
+
+    "not be able to migrate" when {
+      "migration is from V6 to V2" in {
+        migrator.canMigrate(6, 2) shouldBe false
+      }
+
+      "migration is from V6 to V7" in {
+        migrator.canMigrate(6, 7) shouldBe false
+      }
+
+      "migration is from V1 to V7" in {
+        migrator.canMigrate(1, 7) shouldBe false
+      }
+    }
+
+    "migrate forward" when {
+      "migration is from V1 to V5" in {
+        migrator.migrate("V1", 1, 5) shouldBe "V1V2V3V4V5"
+      }
+
+      "migration is from V2 to V5" in {
+        migrator.migrate("V1V2", 2, 5) shouldBe "V1V2V3V4V5"
+      }
+
+      "migration is from V1 to V4" in {
+        migrator.migrate("V1", 1, 4) shouldBe "V1V2V3V4"
+      }
+
+      "migration is from V1 to V1" in {
+        migrator.migrate("V1", 1, 1) shouldBe "V1"
+      }
+
+      "migration is from V2 to V2" in {
+        migrator.migrate("V1V2", 2, 2) shouldBe "V1V2"
+      }
+    }
+
+    "migrate backward" when {
+      "migration is from V2 to V1" in {
+        migrator.migrate("V1V2", 2, 1) shouldBe "V1"
+      }
+
+      "migration is from V5 to V1" in {
+        migrator.migrate("V1V2V3V4V5", 5, 1) shouldBe "V1V5"
+      }
+
+      "migration is from V5 to V2" in {
+        migrator.migrate("V1V2V3V4V5", 5, 2) shouldBe "V1V2V5"
+      }
+
+      "migration is from V5 to V3" in {
+        migrator.migrate("V1V2V3V4V5", 5, 3) shouldBe "V1V2V3V5"
+      }
+    }
+  }
+}

stamina-json/src/main/scala/stamina/json/json.scala

@@ -78,11 +78,13 @@ package json {
   }
 
   private[json] class VnJsonPersister[T: RootJsonFormat: ClassTag, V <: Version: VersionInfo: MigratableVersion](key: String, migrator: JsonMigrator[V]) extends JsonPersister[T, V](key) {
-    override def canUnpersist(p: Persisted): Boolean = p.key == key && migrator.canMigrate(p.version)
+    private val toVersionNumber = implicitly[VersionInfo[V]].versionNumber
+
+    override def canUnpersist(p: Persisted): Boolean = p.key == key && migrator.canMigrate(p.version, toVersionNumber)
 
     def persist(t: T): Persisted = Persisted(key, currentVersion, toJsonBytes(t))
     def unpersist(p: Persisted): T = {
-      if (canUnpersist(p)) migrator.migrate(parseJson(p.bytes), p.version).convertTo[T]
+      if (canUnpersist(p)) migrator.migrate(parseJson(p.bytes), p.version, toVersionNumber).convertTo[T]
       else throw new IllegalArgumentException(cannotUnpersist(p))
     }
   }