Save a list in a property in an entity in xodus

user_1066506 0
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I have not found how to save a general list of primitive types, e.g. ints, or strings, in a property of an entity. I might have missed something obvious...

https://github.com/JetBrains/xodus/wiki/Entity-Stores described that "only Java primitive types, Strings, and ComparableSet values can be used by default".

It seems not hard to convert an Iterable into a ComparableSet. However it is a Set.

I will take a look into PersistentEntityStore.registerCustomPropertyType() to see if that helps. I just feel wrong to do that to just save a list of integers.

Links seemed to be able to serve as a way of saving a list of Entitys. But it seems there is no addProperty() counterpart to addLink().

Appreciated if some one can share a way or a workaround for this, or maybe why this is not supported.

Thanks

xodus
1 Answers

As mentioned in the comments, one workaround I came up with was to create a ComparableList, by adopting code from ComparableSet.

The idea was to make a list that is able to convert to and from an ArrayByteIterable, and register it with .registerCustomPropertyType(). To do that, 2 classes are needed, ComparableList and ComparableListBinding. I'm sharing one iteration I used at the bottom. By the way, I made them immutable, comparing to the mutable ComparableSet. The newly implemented types should be registered once in a transaction of the store before using them.

That should allow you to store and retrieve a list. However the items in ComparableList would not get indexed as they would when saved in a ComparableSet -- there are some special treatment for ComparableSet in the entity store implementation. So without modifying the library, indexing would work only with hacks like creating another property to just index the values.

I was considering to implement a different entity store that could better support lists, on top of the xodus key-value store, bypassing the xodus entity store entirely. That might be a better solution to the list issue we are talking about here.

ComparableList:

@SuppressWarnings("unchecked")
public class ComparableList<T extends Comparable<T>> implements Comparable<ComparableList<T>>,
    Iterable<T> {
  @Nonnull
  private final ImmutableList<T> list;

  public ComparableList(@Nonnull final Iterable<T> iterable) {
    list = ImmutableList.copyOf(iterable);
  }

  @Override
  public int compareTo(@Nonnull final ComparableList<T> other) {
    final Iterator<T> thisIt = list.iterator();
    final Iterator<T> otherIt = other.list.iterator();
    while (thisIt.hasNext() && otherIt.hasNext()) {
      final int cmp = thisIt.next().compareTo(otherIt.next());
      if (cmp != 0) {
        return cmp;
      }
    }
    if (thisIt.hasNext()) {
      return 1;
    }
    if (otherIt.hasNext()) {
      return -1;
    }
    return 0;
  }

  @NotNull
  @Override
  public Iterator<T> iterator() {
    return list.iterator();
  }

  @Nullable
  public Class<T> getItemClass() {
    final Iterator<T> it = list.iterator();
    return it.hasNext() ? (Class<T>) it.next().getClass() : null;
  }

  @Override
  public String toString() {
    return list.toString();
  }
}

ComparableListBinding:

@SuppressWarnings({"unchecked", "rawtypes"})
public class ComparableListBinding extends ComparableBinding {

  public static final ComparableListBinding INSTANCE = new ComparableListBinding();

  private ComparableListBinding() {}

  @Override
  public ComparableList readObject(@NotNull final ByteArrayInputStream stream) {
    final int valueTypeId = stream.read() ^ 0x80;
    final ComparableBinding itemBinding = ComparableValueType.getPredefinedBinding(valueTypeId);
    final ImmutableList.Builder<Comparable> builder = ImmutableList.builder();
    while (stream.available() > 0) {
      builder.add(itemBinding.readObject(stream));
    }
    return new ComparableList(builder.build());
  }

  @Override
  public void writeObject(@NotNull final LightOutputStream output,
      @NotNull final Comparable object) {
    final ComparableList<? extends Comparable> list = (ComparableList) object;
    final Class itemClass = list.getItemClass();
    if (itemClass == null) {
      throw new ExodusException("Attempt to write empty ComparableList");
    }
    final ComparableValueType type = ComparableValueType.getPredefinedType(itemClass);
    output.writeByte(type.getTypeId());
    final ComparableBinding itemBinding = type.getBinding();
    list.forEach(o -> itemBinding.writeObject(output, o));
  }

  /**
   * De-serializes {@linkplain ByteIterable} entry to a {@code ComparableList} value.
   *
   * @param entry {@linkplain ByteIterable} instance
   * @return de-serialized value
   */
  public static ComparableList entryToComparableList(@NotNull final ByteIterable entry) {
    return (ComparableList) INSTANCE.entryToObject(entry);
  }

  /**
   * Serializes {@code ComparableList} value to the {@linkplain ArrayByteIterable} entry.
   *
   * @param object value to serialize
   * @return {@linkplain ArrayByteIterable} entry
   */
  public static ArrayByteIterable comparableSetToEntry(@NotNull final ComparableList object) {
    return INSTANCE.objectToEntry(object);
  }
}
user_10665060
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