Add the ability to clear a playlist.

core/src/main/groovy/com/jdbernard/wdiwtlt/db/ORM.groovy

@@ -116,9 +116,10 @@ public class ORM {
             .append(model.id)
             .toString()
 
-        logger.debug("Updating model.\n\tSQL: {}\n\tPARAMS: {}", query, params)
-        sql.executeUpdate(query, params)
-        return model }
+        return withTransaction {
+            logger.debug("Updating model.\n\tSQL: {}\n\tPARAMS: {}", query, params)
+            sql.executeUpdate(query, params)
+            return refresh(model) } }
 
     public def create(def model) {
         def columns = []
@@ -164,7 +165,7 @@ public class ORM {
         String col1 = nameFromModel(modelClass1.simpleName) + "_id"
         String col2 = nameFromModel(modelClass2.simpleName) + "_id"
 
-        withTransaction {
+        return withTransaction {
             def query = """\
                 SELECT * FROM $linkTable
                 WHERE "${col1}" = ? AND "${col2}" = ?"""
@@ -475,11 +476,24 @@ public class ORM {
 
     public Playlist addToPlaylist(int playlistId, int mediaFileId) {
         return withTransaction {
-            associate(Playlist, MediaFile, p, mf)
             def p = getById(Playlist, playlistId)
+            if (!p) return null
+            associate(Playlist, MediaFile, p, mf)
             p.mediaFileCount++
-            update(p)
-            return refresh(p) } }
+            return update(p) } }
+
+    public Playlist removeAllFromPlaylist(int playlistId) {
+        return withTransaction {
+            def p = getById(Playlist, playlistId)
+            if (!p) return null
+            String query =
+                "DELETE FROM playlists_media_files WHERE playlist_id = ?"
+            def sqlParams = [playlistId]
+            logger.debug("Clearing playlist.\n\tSQL: {}\n\tPARAMS: {}",
+                query, sqlParams)
+            sql.execute(query, sqlParams)
+            p.mediaFileCount = 0
+            return update(p) } }
 
     public List<Playlist> removeEmptyPlaylists() {
         throw new UnsupportedOperationException("Not yet implemented.");
@@ -525,8 +539,8 @@ public class ORM {
             sqlParams << params.name } }
 
     /// ### Utility functions
-    public void withTransaction(Closure c) {
-        try { sql.execute("BEGIN TRANSACTION"); c() }
+    public def withTransaction(Closure c) {
+        try { sql.execute("BEGIN TRANSACTION"); return c() }
         finally { sql.execute("COMMIT") }
     }
     public static String nameToModel(String name) {