SampleMediaCollection.md

so a MediaCollection (or any ContentCollection) has a List.
The ContentElement interface will have a save method, that takes some reference to a database or database helper, and the implementation will take care of saving:

void Chapter.save(Database db) {
  Values values = new Values();
  // populate values with information
  db.insertOrUpdate(CHAPTERS_TABLE).values(values);
}
...
void MediaCollection.save(Database db) {
  for(ContentElement contentElement : contentElements ) {
    contentElement.save(db);
  }
}

That's fairly straightforward, but reading out is not - need to know what table to query, and what id's for that table. Subathra had the idea to maintain a map table:

create table MAP_TABLE (collectionId int, table enum/varchar, referenceId int)

List<ContentElement> MedaiCollection.getElementsFromDb() {
  List<ContentElement> contentElements = new ArrayList<>();
  Cursor cursor = db.select(MAP_TABLE).where("id", getId()).findAll();
  while(cursor.hasNext()){
    String table = cursor.getColumn(1);
    int referenceId = cursor.getColumn(2);
    Cursor elementCursor = db.select(table).where("id", referenceId).findFirst();
    Class<? extends ContentElement> clazz = Class.forName(table);
    ContentElement contentElement = clazz.newInstance();
    contentElement.read(elementCursor);
    contentElements.add(contentElement);
  }
  return contentElements;
}

Which means the save operation now how to save to the mapping table as well...

void MediaCollection.save() {
  Database db = getDb();
  for(ContentElement contentElement : contentElements ) {
    contentElement.save(db);
    db.insert(MAP_TABLE).values(collectionId, contentELement.getClass(), contentElement.getIdentifier());
  }
}

This is all very similar to what Subathra had; the real difference is that we don't have to maintain lists in memory, just in the database.