X-Git-Url: https://git.pterodactylus.net/?a=blobdiff_plain;f=src%2Fmain%2Fjava%2Fnet%2Fpterodactylus%2Fdemoscenemusic%2Fdata%2FDataManager.java;h=f3d35788f2cfaee5af00bef82ef0c4a78227debb;hb=51130ce0f519aa3ac648999413e76558679ee182;hp=214d0c400b8fcd77cd7f84d33ea58533670556ac;hpb=6b67d7d90a31de11c5c1e622b269022112f4fe02;p=demoscenemusic.git

diff --git a/src/main/java/net/pterodactylus/demoscenemusic/data/DataManager.java b/src/main/java/net/pterodactylus/demoscenemusic/data/DataManager.java
index 214d0c4..f3d3578 100644
--- a/src/main/java/net/pterodactylus/demoscenemusic/data/DataManager.java
+++ b/src/main/java/net/pterodactylus/demoscenemusic/data/DataManager.java
@@ -19,18 +19,26 @@ package net.pterodactylus.demoscenemusic.data;
 
 import java.sql.ResultSet;
 import java.sql.SQLException;
-import java.util.ArrayList;
 import java.util.Collection;
+import java.util.EnumMap;
 import java.util.HashSet;
 import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.UUID;
+import java.util.concurrent.Callable;
 
+import net.pterodactylus.demoscenemusic.data.Track.Relationship;
+import net.pterodactylus.util.collection.Memoizer;
 import net.pterodactylus.util.database.Database;
 import net.pterodactylus.util.database.DatabaseException;
 import net.pterodactylus.util.database.Field;
 import net.pterodactylus.util.database.Join;
 import net.pterodactylus.util.database.Join.JoinType;
+import net.pterodactylus.util.database.ObjectCreator;
+import net.pterodactylus.util.database.ObjectCreator.StringCreator;
 import net.pterodactylus.util.database.OrderField;
-import net.pterodactylus.util.database.OrderField.Order;
+import net.pterodactylus.util.database.Parameter.IntegerParameter;
 import net.pterodactylus.util.database.Parameter.StringParameter;
 import net.pterodactylus.util.database.Query;
 import net.pterodactylus.util.database.Query.Type;
@@ -39,103 +47,1257 @@ import net.pterodactylus.util.database.ValueField;
 import net.pterodactylus.util.database.ValueFieldWhereClause;
 
 /**
- * TODO
+ * Interface between the database and the application.
  *
  * @author <a href="mailto:bombe@pterodactylus.net">David ‘Bombe’ Roden</a>
  */
 public class DataManager {
 
+	/** The artist object creator. */
+	@SuppressWarnings("synthetic-access")
+	private final ObjectCreator<Artist> artistCreator = new ArtistCreator();
+
+	/** The group object creator. */
+	@SuppressWarnings("synthetic-access")
+	private final ObjectCreator<Group> groupCreator = new GroupCreator();
+
+	/** The track object creator. */
+	@SuppressWarnings("synthetic-access")
+	private final ObjectCreator<Track> trackCreator = new TrackCreator();
+
+	/** The track object creator. */
+	@SuppressWarnings("synthetic-access")
+	private final ObjectCreator<TrackDerivative> trackDerivativeCreator = new TrackDerivativeCreator();
+
+	/** The style object creator. */
+	@SuppressWarnings("synthetic-access")
+	private final ObjectCreator<Style> styleCreator = new StyleCreator();
+
+	/** The party object creator. */
+	@SuppressWarnings("synthetic-access")
+	private final ObjectCreator<Party> partyCreator = new PartyCreator();
+
+	/** The {@link User} object creator. */
+	@SuppressWarnings("synthetic-access")
+	private final ObjectCreator<User> userCreator = new UserCreator();
+
+	/** The database. */
 	private final Database database;
 
+	/**
+	 * Creates a new data manager.
+	 *
+	 * @param database
+	 *            The database to operate on
+	 */
 	public DataManager(Database database) {
 		this.database = database;
 	}
 
+	/**
+	 * Returns all artists.
+	 *
+	 * @return All artists
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
 	public Collection<Artist> getAllArtists() throws DatabaseException {
 		Query query = new Query(Type.SELECT, "ARTISTS");
 		query.addField(new Field("ARTISTS.*"));
-		query.addField(new Field("GROUP_ARTISTS.*"));
-		query.addField(new Field("GROUPS.*"));
+		return loadArtistProperties(database.getMultiple(query, artistCreator));
+	}
+
+	/**
+	 * Returns the artist with the given ID.
+	 *
+	 * @param id
+	 *            The ID of the artist
+	 * @return The artist with the given ID, or {@code null} if there is no
+	 *         artist with the given ID
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public Artist getArtistById(String id) throws DatabaseException {
+		Query query = new Query(Type.SELECT, "ARTISTS");
+		query.addField(new Field("ARTISTS.*"));
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("ARTISTS.ID", new StringParameter(id))));
+		return loadArtistProperties(database.getSingle(query, artistCreator));
+	}
+
+	/**
+	 * Returns all artists that belong to the group with the given ID.
+	 *
+	 * @param groupId
+	 *            The ID of the group
+	 * @return All artists belonging to the given group
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public Collection<Artist> getArtistsByGroup(String groupId) throws DatabaseException {
+		Query query = new Query(Type.SELECT, "ARTISTS");
+		query.addField(new Field("ARTISTS.*"));
 		query.addJoin(new Join(JoinType.INNER, "GROUP_ARTISTS", new Field("ARTISTS.ID"), new Field("GROUP_ARTISTS.ARTIST")));
-		query.addJoin(new Join(JoinType.INNER, "GROUPS", new Field("GROUP_ARTISTS.GROUP_"), new Field("GROUPS.ID")));
-		query.addOrderField(new OrderField(new Field("ARTISTS.ID")));
-		ArtistLoader artistLoader = new ArtistLoader();
-		database.process(query, artistLoader);
-		return artistLoader.all();
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("GROUP_ARTISTS.GROUP_", new StringParameter(groupId))));
+		return loadArtistProperties(database.getMultiple(query, artistCreator));
 	}
 
-	public Track trackById(String id) throws DatabaseException {
+	/**
+	 * Returns all artists involved in the track with the given ID.
+	 *
+	 * @param trackId
+	 *            The ID of the track
+	 * @return All artists involved in the track, in preferred order
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public List<Artist> getArtistsByTrack(String trackId) throws DatabaseException {
+		Query query = new Query(Type.SELECT, "ARTISTS");
+		query.addField(new Field("ARTISTS.*"));
+		query.addJoin(new Join(JoinType.INNER, "TRACK_ARTISTS", new Field("TRACK_ARTISTS.ARTIST"), new Field("ARTISTS.ID")));
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("TRACK_ARTISTS.TRACK", new StringParameter(trackId))));
+		query.addOrderField(new OrderField(new Field("TRACK_ARTISTS.DISPLAY_ORDER")));
+		return loadArtistProperties(database.getMultiple(query, artistCreator));
+	}
+
+	/**
+	 * Creates a new artist with the given name.
+	 *
+	 * @param name
+	 *            The name of the artist
+	 * @return The created artist
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public Artist createArtist(String name) throws DatabaseException {
+		Query query = new Query(Type.INSERT, "ARTISTS");
+		String id = UUID.randomUUID().toString();
+		query.addValueField(new ValueField("ID", new StringParameter(id)));
+		query.addValueField(new ValueField("NAME", new StringParameter(name)));
+		database.insert(query);
+		return loadArtistProperties(getArtistById(id));
+	}
+
+	/**
+	 * Saves the given artist.
+	 *
+	 * @param artist
+	 *            The artist to save
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public void saveArtist(Artist artist) throws DatabaseException {
+		Query query = new Query(Type.UPDATE, "ARTISTS");
+		query.addValueField(new ValueField("NAME", new StringParameter(artist.getName())));
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("ID", new StringParameter(artist.getId()))));
+		database.update(query);
+		saveArtistProperties(artist);
+	}
+
+	/**
+	 * Saves the properties of the given artist.
+	 *
+	 * @param artist
+	 *            The artist whose properties to save
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public void saveArtistProperties(Artist artist) throws DatabaseException {
+		saveProperties(artist.getProperties(), "ARTIST_PROPERTIES", "ARTIST", artist.getId());
+	}
+
+	/**
+	 * Saves the given properties to the given table for the given principal.
+	 *
+	 * @param properties
+	 *            The properties to save
+	 * @param table
+	 *            The table in which to save the properties
+	 * @param type
+	 *            The type of the principal (e. g. “ARTIST” or “TRACK”)
+	 * @param id
+	 *            The ID of the principial
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public void saveProperties(Properties properties, String table, String type, String id) throws DatabaseException {
+		if (!properties.isDirty()) {
+			return;
+		}
+		Query query = new Query(Type.DELETE, table);
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField(type, new StringParameter(id))));
+		database.update(query);
+		for (Entry<String, String> property : properties) {
+			Query insertQuery = new Query(Type.INSERT, table);
+			insertQuery.addValueField(new ValueField(type, new StringParameter(id)));
+			insertQuery.addValueField(new ValueField("PROPERTY", new StringParameter(property.getKey())));
+			insertQuery.addValueField(new ValueField("VALUE", new StringParameter(property.getValue())));
+			database.insert(insertQuery);
+		}
+		properties.resetDirty();
+	}
+
+	/**
+	 * Loads the properties for an artist.
+	 *
+	 * @param artist
+	 *            The artist to load the properties for
+	 * @return The artist
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public Artist loadArtistProperties(final Artist artist) throws DatabaseException {
+		return loadProperties(artist, "ARTIST_PROPERTIES", "ARTIST");
+	}
+
+	/**
+	 * Loads the properties of all given artists.
+	 *
+	 * @param artists
+	 *            The artists to load the properties for
+	 * @return The list of artists
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public List<Artist> loadArtistProperties(List<Artist> artists) throws DatabaseException {
+		for (Artist artist : artists) {
+			loadArtistProperties(artist);
+		}
+		return artists;
+	}
+
+	/**
+	 * Returns all remix artists involved in the track with the given ID.
+	 *
+	 * @param trackId
+	 *            The ID of the track
+	 * @return All remix artists involved in the track, in preferred order
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public List<Artist> getRemixArtistsByTrack(String trackId) throws DatabaseException {
+		Query query = new Query(Type.SELECT, "ARTISTS");
+		query.addField(new Field("ARTISTS.*"));
+		query.addJoin(new Join(JoinType.INNER, "TRACK_REMIX_ARTISTS", new Field("TRACK_REMIX_ARTISTS.ARTIST"), new Field("ARTISTS.ID")));
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("TRACK_REMIX_ARTISTS.TRACK", new StringParameter(trackId))));
+		query.addOrderField(new OrderField(new Field("TRACK_REMIX_ARTISTS.DISPLAY_ORDER")));
+		return database.getMultiple(query, artistCreator);
+	}
+
+	/**
+	 * Returns all related tracks for the track with the given ID.
+	 *
+	 * @param trackId
+	 *            The ID of the tracks
+	 * @return A mapping from relationship to all tracks that match the relation
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public Map<Relationship, Collection<Track>> getRelatedTracksByTrack(String trackId) throws DatabaseException {
+		Query query = new Query(Type.SELECT, "TRACKS");
+		query.addField(new Field("TRACKS.*"));
+		query.addField(new Field("TRACK_RELATIONS.*"));
+		query.addJoin(new Join(JoinType.INNER, "TRACK_RELATIONS", new Field("TRACK_RELATIONS.TRACK"), new Field("TRACK_RELATIONS.RELATED_TRACK")));
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("TRACK_RELATIONS.TRACK", new StringParameter(trackId))));
+		final Map<Relationship, Collection<Track>> relatedTracks = new EnumMap<Relationship, Collection<Track>>(Relationship.class);
+		database.process(query, new ResultProcessor() {
+
+			@Override
+			@SuppressWarnings("synthetic-access")
+			public void processResult(ResultSet resultSet) throws SQLException {
+				Track track = trackCreator.createObject(resultSet);
+				Relationship relationship = Relationship.valueOf(resultSet.getString("TRACK_RELATIONS.RELATIONSHIP"));
+				if (!relatedTracks.containsKey(relationship)) {
+					relatedTracks.put(relationship, new HashSet<Track>());
+				}
+				relatedTracks.get(relationship).add(track);
+			}
+		});
+		return relatedTracks;
+	}
+
+	/**
+	 * Returns the track with the given ID.
+	 *
+	 * @param id
+	 *            The ID of the track
+	 * @return The track with the given ID, or {@code null} if there is no such
+	 *         track
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public Track getTrackById(String id) throws DatabaseException {
 		Query query = new Query(Type.SELECT, "TRACKS");
 		query.addField(new Field("TRACKS.*"));
-		query.addField(new Field("TRACK_ARTISTS.*"));
-		query.addField(new Field("ARTISTS.*"));
-		query.addField(new Field("TRACK_STYLES.*"));
-		query.addField(new Field("STYLES.*"));
-		query.addJoin(new Join(JoinType.INNER, "TRACK_ARTISTS", new Field("TRACKS.ID"), new Field("TRACK_ARTISTS.TRACK")));
-		query.addJoin(new Join(JoinType.INNER, "ARTISTS", new Field("TRACK_ARTISTS.ARTIST"), new Field("ARTISTS.ID")));
-		query.addJoin(new Join(JoinType.INNER, "TRACK_STYLES", new Field("TRACKS.ID"), new Field("TRACK_STYLES.TRACK")));
-		query.addJoin(new Join(JoinType.INNER, "STYLES", new Field("TRACK_STYLES.STYLE"), new Field("STYLES.ID")));
 		query.addWhereClause(new ValueFieldWhereClause(new ValueField("TRACKS.ID", new StringParameter(id))));
-		query.addOrderField(new OrderField(new Field("TRACKS.ID"), Order.ASCENDING));
-		query.addOrderField(new OrderField(new Field("TRACK_ARTISTS.DISPLAY_ORDER"), Order.ASCENDING));
-		TrackLoader trackLoader = new TrackLoader();
-		database.process(query, trackLoader);
-		return trackLoader.get();
+		return loadTrackProperties(database.getSingle(query, trackCreator));
+	}
+
+	/**
+	 * Returns the track that contains the derivative with the given ID.
+	 *
+	 * @param id
+	 *            The ID of the track derivative
+	 * @return The track the derivative belongs to, or {@code null} if there is
+	 *         no such track
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public Track getTrackByDerivativeId(String id) throws DatabaseException {
+		Query query = new Query(Type.SELECT, "TRACKS");
+		query.addField(new Field("TRACKS.*"));
+		query.addJoin(new Join(JoinType.INNER, "TRACK_DERIVATIVES", new Field("TRACK_DERIVATIVES.TRACK"), new Field("TRACKS.ID")));
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("TRACK_DERIVATIVES.ID", new StringParameter(id))));
+		return loadTrackProperties(database.getSingle(query, trackCreator));
+	}
+
+	/**
+	 * Returns all tracks by the artist with the given ID.
+	 *
+	 * @param artistId
+	 *            The ID of the artist
+	 * @return All tracks by the given artist
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public Collection<Track> getTracksByArtist(String artistId) throws DatabaseException {
+		Query query = new Query(Type.SELECT, "TRACKS");
+		query.addField(new Field("TRACKS.*"));
+		query.addJoin(new Join(JoinType.INNER, "TRACK_ARTISTS", new Field("TRACKS.ID"), new Field("TRACK_ARTISTS.TRACK")));
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("TRACK_ARTISTS.ARTIST", new StringParameter(artistId))));
+		return loadTrackProperties(database.getMultiple(query, trackCreator));
+	}
+
+	/**
+	 * Returns all tracks that were released at the party with the given ID.
+	 *
+	 * @param partyId
+	 *            The ID of the party
+	 * @return All tracks that were released at the given party
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public Collection<Track> getTracksByParty(String partyId) throws DatabaseException {
+		Query query = new Query(Type.SELECT, "TRACKS");
+		query.addField(new Field("TRACKS.*"));
+		query.addJoin(new Join(JoinType.INNER, "PARTY_TRACKS", new Field("PARTY_TRACKS.TRACK"), new Field("TRACKS.ID")));
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("PARTY_TRACKS.PARTY", new StringParameter(partyId))));
+		return loadTrackProperties(database.getMultiple(query, trackCreator));
+	}
+
+	/**
+	 * Loads the properties for the given track.
+	 *
+	 * @param track
+	 *            The track for which to load the properties
+	 * @return The track with the properties loaded
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public Track loadTrackProperties(Track track) throws DatabaseException {
+		return loadProperties(track, "TRACK_PROPERTIES", "TRACK");
+	}
+
+	/**
+	 * Loads the properties for the given tracks.
+	 *
+	 * @param tracks
+	 *            The tracks for which to load the properties
+	 * @return The tracks with the properties loaded
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public List<Track> loadTrackProperties(List<Track> tracks) throws DatabaseException {
+		for (Track track : tracks) {
+			loadTrackProperties(track);
+		}
+		return tracks;
+	}
+
+	/**
+	 * Creates a new track with the given name.
+	 *
+	 * @param name
+	 *            The name of the track
+	 * @return The created track
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public Track createTrack(String name) throws DatabaseException {
+		Query query = new Query(Type.INSERT, "TRACKS");
+		String id = UUID.randomUUID().toString();
+		query.addValueField(new ValueField("ID", new StringParameter(id)));
+		query.addValueField(new ValueField("NAME", new StringParameter(name)));
+		database.insert(query);
+		return getTrackById(id);
+	}
+
+	/**
+	 * Saves the given track. This also saves all relationships of the track.
+	 *
+	 * @param track
+	 *            The track to save
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public void saveTrack(Track track) throws DatabaseException {
+		Query query = new Query(Type.UPDATE, "TRACKS");
+		query.addValueField(new ValueField("TRACKS.NAME", new StringParameter(track.getName())));
+		query.addValueField(new ValueField("TRACKS.REMIX", new StringParameter(track.getRemix())));
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("TRACKS.ID", new StringParameter(track.getId()))));
+		database.update(query);
+		/* store artist information. */
+		track.getArtists(); /* prefetch artists. */
+		query = new Query(Type.DELETE, "TRACK_ARTISTS");
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("TRACK_ARTISTS.TRACK", new StringParameter(track.getId()))));
+		database.update(query);
+		for (int index = 0; index < track.getArtists().size(); ++index) {
+			query = new Query(Type.INSERT, "TRACK_ARTISTS");
+			query.addValueField(new ValueField("TRACK_ARTISTS.TRACK", new StringParameter(track.getId())));
+			query.addValueField(new ValueField("TRACK_ARTISTS.ARTIST", new StringParameter(track.getArtists().get(index).getId())));
+			query.addValueField(new ValueField("TRACK_ARTISTS.DISPLAY_ORDER", new IntegerParameter(index + 1)));
+			database.insert(query);
+		}
+
+		/* store party links. */
+		Collection<Party> parties = track.getParties(); /* prefetch parties. */
+		query = new Query(Type.DELETE, "PARTY_TRACKS");
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("PARTY_TRACKS.TRACK", new StringParameter(track.getId()))));
+		database.update(query);
+		for (Party party : parties) {
+			query = new Query(Type.INSERT, "PARTY_TRACKS");
+			query.addValueField(new ValueField("PARTY_TRACKS.TRACK", new StringParameter(track.getId())));
+			query.addValueField(new ValueField("PARTY_TRACKS.PARTY", new StringParameter(party.getId())));
+			database.insert(query);
+		}
+
+		/* store properties. */
+		saveProperties(track.getProperties(), "TRACK_PROPERTIES", "TRACK", track.getId());
+	}
+
+	/**
+	 * Returns the derivative with the given ID.
+	 *
+	 * @param id
+	 *            The ID of the derivatives to load
+	 * @return The derivative with the given ID
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public TrackDerivative getTrackDerivativeById(String id) throws DatabaseException {
+		Query query = new Query(Type.SELECT, "TRACK_DERIVATIVES");
+		query.addField(new Field("TRACK_DERIVATIVES.*"));
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("TRACK_DERIVATIVES.ID", new StringParameter(id))));
+		return loadTrackDerivativeProperties(database.getSingle(query, trackDerivativeCreator));
+	}
+
+	/**
+	 * Returns the derivatives for the given track.
+	 *
+	 * @param trackId
+	 *            The track ID to get the derivatives for
+	 * @return The derivatives for the given track
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public Collection<TrackDerivative> getTrackDerivativesByTrack(String trackId) throws DatabaseException {
+		Query query = new Query(Type.SELECT, "TRACK_DERIVATIVES");
+		query.addField(new Field("TRACK_DERIVATIVES.*"));
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("TRACK_DERIVATIVES.TRACK", new StringParameter(trackId))));
+		return loadTrackDerivativeProperties(database.getMultiple(query, trackDerivativeCreator));
+	}
+
+	/**
+	 * Creates a new derivative for the given track.
+	 *
+	 * @param track
+	 *            The track to create the derivative for
+	 * @return The created derivative
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public TrackDerivative createTrackDerivative(Track track) throws DatabaseException {
+		Query query = new Query(Type.INSERT, "TRACK_DERIVATIVES");
+		String id = UUID.randomUUID().toString();
+		query.addValueField(new ValueField("TRACK_DERIVATIVES.ID", new StringParameter(id)));
+		query.addValueField(new ValueField("TRACK_DERIVATIVES.TRACK", new StringParameter(track.getId())));
+		database.insert(query);
+		return getTrackDerivativeById(id);
+	}
+
+	/**
+	 * Loads the properties for the given track derivative.
+	 *
+	 * @param trackDerivative
+	 *            The track derivative to load the properties for
+	 * @return The track derivative with its properties loaded
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public TrackDerivative loadTrackDerivativeProperties(TrackDerivative trackDerivative) throws DatabaseException {
+		return loadProperties(trackDerivative, "TRACK_DERIVATIVE_PROPERTIES", "TRACK_DERIVATIVE");
+	}
+
+	/**
+	 * Loads the properties for the given track derivatives.
+	 *
+	 * @param trackDerivatives
+	 *            The track derivatives to load the properties for
+	 * @return The track derivatives with their properties loaded
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public List<TrackDerivative> loadTrackDerivativeProperties(List<TrackDerivative> trackDerivatives) throws DatabaseException {
+		for (TrackDerivative trackDerivative : trackDerivatives) {
+			loadTrackDerivativeProperties(trackDerivative);
+		}
+		return trackDerivatives;
+	}
+
+	/**
+	 * Saves the given track derivative. As a track derivative does not have any
+	 * attributes of its own only its properties are saved.
+	 *
+	 * @param trackDerivative
+	 *            The track derivative to save
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public void saveTrackDerivate(TrackDerivative trackDerivative) throws DatabaseException {
+		saveProperties(trackDerivative.getProperties(), "TRACK_DERIVATIVE_PROPERTIES", "TRACK_DERIVATIVE", trackDerivative.getId());
+	}
+
+	/**
+	 * Removes the given track derivative and all its properties from the
+	 * database.
+	 *
+	 * @param trackDerivative
+	 *            The track derivative to remove
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public void removeTrackDerivative(TrackDerivative trackDerivative) throws DatabaseException {
+		Query query = new Query(Type.DELETE, "TRACK_DERIVATIVES");
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("TRACK_DERIVATIVES.ID", new StringParameter(trackDerivative.getId()))));
+		database.update(query);
+		/* remove the properties. */
+		saveProperties(new Properties().set("dirty", "true").removeAll(), "TRACK_DERIVATIVE_PROPERTIES", "TRACK_DERIVATIVE", trackDerivative.getId());
+	}
+
+	/**
+	 * Returns all groups the artist with the given ID belongs to.
+	 *
+	 * @param artistId
+	 *            The ID of the artist
+	 * @return All groups the artist belongs to
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public Collection<Group> getGroupsByArtist(String artistId) throws DatabaseException {
+		Query query = new Query(Type.SELECT, "GROUPS");
+		query.addField(new Field("GROUPS.*"));
+		query.addJoin(new Join(JoinType.INNER, "GROUP_ARTISTS", new Field("GROUPS.ID"), new Field("GROUP_ARTISTS.GROUP_")));
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("GROUP_ARTISTS.ARTIST", new StringParameter(artistId))));
+		return database.getMultiple(query, groupCreator);
+	}
+
+	/**
+	 * Returns all styles for the track with the given ID.
+	 *
+	 * @param trackId
+	 *            The ID of the track
+	 * @return All styles for the given track
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public Collection<Style> getStylesByTrack(String trackId) throws DatabaseException {
+		Query query = new Query(Type.SELECT, "STYLES");
+		query.addField(new Field("STYLES.*"));
+		query.addJoin(new Join(JoinType.INNER, "TRACK_STYLES", new Field("STYLES.ID"), new Field("TRACK_STYLES.STYLE")));
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("TRACK_STYLES.TRACK", new StringParameter(trackId))));
+		return database.getMultiple(query, styleCreator);
+	}
+
+	/**
+	 * Returns all parties.
+	 *
+	 * @return All parties
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public Collection<Party> getAllParties() throws DatabaseException {
+		Query query = new Query(Type.SELECT, "PARTIES");
+		query.addField(new Field("PARTIES.*"));
+		return loadPartyProperties(database.getMultiple(query, partyCreator));
+	}
+
+	/**
+	 * Returns all parties that the track with the given ID was released at.
+	 *
+	 * @param trackId
+	 *            The ID of the track
+	 * @return All parties the track was released at
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public Collection<Party> getPartiesByTrackId(String trackId) throws DatabaseException {
+		Query query = new Query(Type.SELECT, "PARTIES");
+		query.addField(new Field("PARTIES.*"));
+		query.addJoin(new Join(JoinType.INNER, "PARTY_TRACKS", new Field("PARTY_TRACKS.PARTY"), new Field("PARTIES.ID")));
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("PARTY_TRACKS.TRACK", new StringParameter(trackId))));
+		return loadPartyProperties(database.getMultiple(query, partyCreator));
+	}
+
+	/**
+	 * Returns the party with the given ID.
+	 *
+	 * @param partyId
+	 *            The ID of the party
+	 * @return The party with the given ID
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public Party getPartyById(String partyId) throws DatabaseException {
+		Query query = new Query(Type.SELECT, "PARTIES");
+		query.addField(new Field("PARTIES.*"));
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("PARTIES.ID", new StringParameter(partyId))));
+		return loadPartyProperties(database.getSingle(query, partyCreator));
+	}
+
+	/**
+	 * Loads the properties of the given party.
+	 *
+	 * @param party
+	 *            The party to load the properties for
+	 * @return The party with its properties loaded
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public Party loadPartyProperties(Party party) throws DatabaseException {
+		return loadProperties(party, "PARTY_PROPERTIES", "PARTY");
+	}
+
+	/**
+	 * Loads the properties of the given parties.
+	 *
+	 * @param parties
+	 *            The parties to load the properties for
+	 * @return The parties with their properties loaded
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public List<Party> loadPartyProperties(List<Party> parties) throws DatabaseException {
+		for (Party party : parties) {
+			loadPartyProperties(party);
+		}
+		return parties;
+	}
+
+	/**
+	 * Saves the given party.
+	 *
+	 * @param party
+	 *            The party to save
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public void saveParty(Party party) throws DatabaseException {
+		Query query = new Query(Type.UPDATE, "PARTIES");
+		query.addValueField(new ValueField("NAME", new StringParameter(party.getName())));
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("ID", new StringParameter(party.getId()))));
+		database.update(query);
+		savePartyProperties(party);
+	}
+
+	/**
+	 * Saves the properties of the given party.
+	 *
+	 * @param party
+	 *            The party whose properties to save
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public void savePartyProperties(Party party) throws DatabaseException {
+		saveProperties(party.getProperties(), "PARTY_PROPERTIES", "PARTY", party.getId());
+	}
+
+	/**
+	 * Creates a new party with the given name.
+	 *
+	 * @param name
+	 *            The name of the party
+	 * @return The new party
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public Party createParty(String name) throws DatabaseException {
+		Query query = new Query(Type.INSERT, "PARTIES");
+		String id = UUID.randomUUID().toString();
+		query.addValueField(new ValueField("ID", new StringParameter(id)));
+		query.addValueField(new ValueField("NAME", new StringParameter(name)));
+		database.insert(query);
+		return getPartyById(id);
+	}
+
+	/**
+	 * Returns the user with the given name.
+	 *
+	 * @param username
+	 *            The name of the user
+	 * @return The user, or {@code null} if the user does not exist
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public User getUserByName(String username) throws DatabaseException {
+		Query query = new Query(Type.SELECT, "USERS");
+		query.addField(new Field("USERS.*"));
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("USERS.NAME", new StringParameter(username))));
+		return database.getSingle(query, userCreator);
+	}
+
+	/**
+	 * Returns the user connected with the given OpenID.
+	 *
+	 * @param openId
+	 *            The OpenID to find the user for
+	 * @return The user connected with the given OpenID, or {@code null} if
+	 *         there is no such user
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public User getUserByOpenId(String openId) throws DatabaseException {
+		Query query = new Query(Type.SELECT, "USERS");
+		query.addField(new Field("USERS.*"));
+		query.addJoin(new Join(JoinType.INNER, "USER_OPENIDS", new Field("USER_OPENIDS.USER"), new Field("USERS.ID")));
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("USER_OPENIDS.OPENID", new StringParameter(openId))));
+		return database.getSingle(query, userCreator);
+	}
+
+	/**
+	 * Returns all OpenIDs connected with the user with the given ID.
+	 *
+	 * @param userId
+	 *            The ID of the user
+	 * @return All OpenIDs connected with the given user
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	public Collection<String> getOpenIdsByUser(String userId) throws DatabaseException {
+		Query query = new Query(Type.SELECT, "USER_OPENIDS");
+		query.addField(new Field("USER_OPENIDS.*"));
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField("USER_OPENIDS.USER", new StringParameter(userId))));
+		return database.getMultiple(query, new StringCreator("USER_OPENIDS.OPENID"));
+	}
+
+	//
+	// PRIVATE METHODS
+	//
+
+	/**
+	 * Loads the properties for the given object.
+	 *
+	 * @param <T>
+	 *            The type of the object
+	 * @param object
+	 *            The object
+	 * @param table
+	 *            The table to load the properties from
+	 * @param type
+	 *            The type of the object (“ARTIST,” “TRACK,” etc.)
+	 * @return The object with its properties loaded
+	 * @throws DatabaseException
+	 *             if a database error occurs
+	 */
+	private <T extends Base> T loadProperties(final T object, final String table, String type) throws DatabaseException {
+		if (object == null) {
+			return null;
+		}
+		Query query = new Query(Type.SELECT, table);
+		query.addField(new Field(table + ".PROPERTY"));
+		query.addField(new Field(table + ".VALUE"));
+		query.addWhereClause(new ValueFieldWhereClause(new ValueField(type, new StringParameter(object.getId()))));
+		database.process(query, new ResultProcessor() {
+
+			@Override
+			public void processResult(ResultSet resultSet) throws SQLException {
+				if (resultSet.isFirst()) {
+					object.getProperties().removeAll();
+				}
+				object.getProperties().set(resultSet.getString(table + ".PROPERTY"), resultSet.getString(table + ".VALUE"));
+			}
+
+		});
+		return object;
+	}
+
+	/**
+	 * {@link Artist} implementation that retrieves some attributes (such as
+	 * {@link #getGroups()}, and {@link #getTracks()}) from the
+	 * {@link DataManager} on demand.
+	 *
+	 * @author <a href="mailto:bombe@pterodactylus.net">David ‘Bombe’ Roden</a>
+	 */
+	private class LazyArtist extends DefaultArtist {
+
+		/** Memoizer for the tracks by this artist. */
+		private final Memoizer<Void> tracksMemoizer = new Memoizer<Void>(new Callable<Void>() {
+
+			@Override
+			public Void call() throws DatabaseException {
+				if (!hasValue("tracks")) {
+					getValue("tracks", Collection.class).set(getTracksByArtist(getId()));
+				}
+				return null;
+			}
+		});
+
+		/** Memoizer for the groups of this artist. */
+		private final Memoizer<Void> groupsMemoizer = new Memoizer<Void>(new Callable<Void>() {
+
+			@Override
+			public Void call() throws Exception {
+				if (!hasValue("groups")) {
+					getValue("groups", Collection.class).set(getGroupsByArtist(getId()));
+				}
+				return null;
+			}
+
+		});
+
+		/**
+		 * Creates a new lazy artist.
+		 *
+		 * @param id
+		 *            The ID of the artist
+		 */
+		public LazyArtist(String id) {
+			super(id);
+		}
+
+		//
+		// DEFAULTARTIST METHODS
+		//
+
+		/**
+		 * {@inheritDoc}
+		 */
+		@Override
+		public Collection<Group> getGroups() {
+			groupsMemoizer.get();
+			return super.getGroups();
+		}
+
+		/**
+		 * {@inheritDoc}
+		 */
+		@Override
+		public Collection<Track> getTracks() {
+			tracksMemoizer.get();
+			return super.getTracks();
+		}
+
 	}
 
-	private class TrackLoader implements ResultProcessor {
+	/**
+	 * {@link ObjectCreator} implementation that can create {@link Artist}
+	 * objects. This specific class actually creates {@link LazyArtist}
+	 * instances.
+	 *
+	 * @author <a href="mailto:bombe@pterodactylus.net">David ‘Bombe’ Roden</a>
+	 */
+	private class ArtistCreator implements ObjectCreator<Artist> {
 
-		private final List<Track> loadedTracks = new ArrayList<Track>();
-		private Track track;
+		/**
+		 * {@inheritDoc}
+		 */
+		@Override
+		public Artist createObject(ResultSet resultSet) throws SQLException {
+			return new LazyArtist(resultSet.getString("ARTISTS.ID")).setName(resultSet.getString("ARTISTS.NAME"));
+		}
+
+	}
+
+	/**
+	 * {@link Group} implementation that retrieves some attributes (such as
+	 * {@link #getArtists()}) from the {@link DataManager} on demand.
+	 *
+	 * @author <a href="mailto:bombe@pterodactylus.net">David ‘Bombe’ Roden</a>
+	 */
+	private class LazyGroup extends DefaultGroup {
+
+		/** Memoizer for the artist. */
+		private final Memoizer<Void> artistsMemoizer = new Memoizer<Void>(new Callable<Void>() {
+
+			@Override
+			public Void call() throws Exception {
+				if (!hasValue("artists")) {
+					getValue("artists", Collection.class).set(getArtistsByGroup(getId()));
+				}
+				return null;
+			}
+
+		});
+
+		/**
+		 * Creates a new lazy group.
+		 *
+		 * @param id
+		 *            The ID of the group
+		 */
+		public LazyGroup(String id) {
+			super(id);
+		}
+
+		//
+		// DEFAULTGROUP METHODS
+		//
+
+		/**
+		 * {@inheritDoc}
+		 */
+		@Override
+		public Collection<Artist> getArtists() {
+			artistsMemoizer.get();
+			return super.getArtists();
+		}
+
+	}
+
+	/**
+	 * {@link ObjectCreator} implementation that can create {@link Group}
+	 * objects. This specific implementation creates {@link LazyGroup}
+	 * instances.
+	 *
+	 * @author <a href="mailto:bombe@pterodactylus.net">David ‘Bombe’ Roden</a>
+	 */
+	private class GroupCreator implements ObjectCreator<Group> {
 
 		/**
 		 * {@inheritDoc}
 		 */
-		public void processResult(ResultSet resultSet) throws SQLException {
-			String trackId = resultSet.getString("TRACKS.ID");
-			if ((track == null) || (!track.id().equals(trackId))) {
-				track = new DefaultTrack(trackId).name(resultSet.getString("TRACKS.NAME")).artists(new ArrayList<Artist>()).styles(new HashSet<Style>());
-				loadedTracks.add(track);
+		@Override
+		public Group createObject(ResultSet resultSet) throws SQLException {
+			return new LazyGroup(resultSet.getString("GROUPS.ID")).setName(resultSet.getString("GROUPS.NAME")).setUrl(resultSet.getString("GROUPS.URL"));
+		}
+
+	}
+
+	/**
+	 * {@link Track} implementation that retrieves some attributes (such as
+	 * {@link #getArtists()}, and {@link #getStyles()}) from the
+	 * {@link DataManager} on demand.
+	 *
+	 * @author <a href="mailto:bombe@pterodactylus.net">David ‘Bombe’ Roden</a>
+	 */
+	private class LazyTrack extends DefaultTrack {
+
+		/** Memoizer for the artists. */
+		private final Memoizer<Void> artistsMemoizer = new Memoizer<Void>(new Callable<Void>() {
+
+			@Override
+			public Void call() throws Exception {
+				if (!hasValue("artists")) {
+					getValue("artists", List.class).set(getArtistsByTrack(getId()));
+				}
+				return null;
+			}
+
+		});
+
+		/** Memoizer for the styles. */
+		private final Memoizer<Void> stylesMemoizer = new Memoizer<Void>(new Callable<Void>() {
+
+			@Override
+			public Void call() throws Exception {
+				if (!hasValue("styles")) {
+					getValue("styles", Collection.class).set(getStylesByTrack(getId()));
+				}
+				return null;
+			}
+
+		});
+
+		/** Memoizer for the remix artists. */
+		private final Memoizer<Void> remixArtistsMemoizer = new Memoizer<Void>(new Callable<Void>() {
+
+			@Override
+			public Void call() throws Exception {
+				if (!hasValue("remixArtists")) {
+					getValue("remixArtists", List.class).set(getRemixArtistsByTrack(getId()));
+				}
+				return null;
 			}
-			Artist artist = new DefaultArtist(resultSet.getString("ARTISTS.ID")).name(resultSet.getString("ARTISTS.NAME"));
-			if (!track.artists().contains(artist)) {
-				track.artists().add(artist);
+
+		});
+
+		/** Memoizer for the track derivatives. */
+		private final Memoizer<Void> derivativesMemoizer = new Memoizer<Void>(new Callable<Void>() {
+
+			@Override
+			public Void call() throws Exception {
+				if (!hasValue("derivatives")) {
+					getValue("derivatives", Collection.class).set(getTrackDerivativesByTrack(getId()));
+				}
+				return null;
 			}
-			Style style = new DefaultStyle(resultSet.getString("STYLES.ID")).name(resultSet.getString("STYLES.NAME"));
-			track.styles().add(style);
+
+		});
+
+		/** Memoizer for the related tracks. */
+		private final Memoizer<Void> relatedTracksMemoizer = new Memoizer<Void>(new Callable<Void>() {
+
+			@Override
+			public Void call() throws Exception {
+				if (!hasValue("relatedTracks")) {
+					getValue("relatedTracks", Map.class).set(getRelatedTracksByTrack(getId()));
+				}
+				return null;
+			}
+
+		});
+
+		/**
+		 * Creates a new track.
+		 *
+		 * @param id
+		 *            The ID of the track
+		 */
+		public LazyTrack(String id) {
+			super(id);
+		}
+
+		//
+		// DEFAULTTRACK METHODS
+		//
+
+		/**
+		 * {@inheritDoc}
+		 */
+		@Override
+		public List<Artist> getArtists() {
+			artistsMemoizer.get();
+			return super.getArtists();
 		}
 
-		public Track get() {
-			return loadedTracks.isEmpty() ? null : loadedTracks.get(0);
+		/**
+		 * {@inheritDoc}
+		 */
+		@Override
+		public Collection<Style> getStyles() {
+			stylesMemoizer.get();
+			return super.getStyles();
+		}
+
+		/**
+		 * {@inheritDoc}
+		 */
+		@Override
+		public List<Artist> getRemixArtists() {
+			remixArtistsMemoizer.get();
+			return super.getRemixArtists();
+		}
+
+		/**
+		 * {@inheritDoc}
+		 */
+		@Override
+		public Collection<TrackDerivative> getDerivatives() {
+			derivativesMemoizer.get();
+			return super.getDerivatives();
+		}
+
+		/**
+		 * {@inheritDoc}
+		 */
+		@Override
+		public Map<Relationship, Collection<Track>> getRelatedTracks() {
+			relatedTracksMemoizer.get();
+			return super.getRelatedTracks();
+		}
+
+		/**
+		 * {@inheritDoc}
+		 */
+		@Override
+		public Collection<Party> getParties() {
+			if (!hasValue("parties")) {
+				try {
+					getValue("parties", Collection.class).set(getPartiesByTrackId(getId()));
+				} catch (DatabaseException de1) {
+					throw new RuntimeException("Could not load Parties for Track " + getId() + ".", de1);
+				}
+			}
+			return super.getParties();
+		}
+
+	}
+
+	/**
+	 * {@link ObjectCreator} implementation that can create {@link Track}
+	 * objects. This specific implementation creates {@link LazyTrack}
+	 * instances.
+	 *
+	 * @author <a href="mailto:bombe@pterodactylus.net">David ‘Bombe’ Roden</a>
+	 */
+	private class TrackCreator implements ObjectCreator<Track> {
+
+		/**
+		 * {@inheritDoc}
+		 */
+		@Override
+		public Track createObject(ResultSet resultSet) throws SQLException {
+			return new LazyTrack(resultSet.getString("TRACKS.ID")).setName(resultSet.getString("TRACKS.NAME")).setRemix(resultSet.getString("TRACKS.REMIX"));
+		}
+
+	}
+
+	/**
+	 * {@link ObjectCreator} implementation that can create
+	 * {@link TrackDerivative} objects.
+	 *
+	 * @author <a href="mailto:bombe@pterodactylus.net">David ‘Bombe’ Roden</a>
+	 */
+	private class TrackDerivativeCreator implements ObjectCreator<TrackDerivative> {
+
+		/**
+		 * {@inheritDoc}
+		 */
+		@Override
+		public TrackDerivative createObject(ResultSet resultSet) throws SQLException {
+			return new DefaultTrackDerivative(resultSet.getString("TRACK_DERIVATIVES.ID"));
 		}
 
 	}
 
-	private class ArtistLoader implements ResultProcessor {
+	/**
+	 * {@link ObjectCreator} implementation that can create {@link Style}
+	 * objects.
+	 *
+	 * @author <a href="mailto:bombe@pterodactylus.net">David ‘Bombe’ Roden</a>
+	 */
+	private class StyleCreator implements ObjectCreator<Style> {
+
+		/**
+		 * {@inheritDoc}
+		 */
+		@Override
+		public Style createObject(ResultSet resultSet) throws SQLException {
+			return new DefaultStyle(resultSet.getString("STYLES.ID")).setName(resultSet.getString("STYLES.NAME"));
+		}
+
+	}
 
-		private final List<Artist> artists = new ArrayList<Artist>();
-		private Artist artist;
+	/**
+	 * {@link Party} implementation that loads additional information only on
+	 * demand.
+	 *
+	 * @author <a href="mailto:bombe@pterodactylus.net">David ‘Bombe’ Roden</a>
+	 */
+	private class LazyParty extends DefaultParty {
 
-		public List<Artist> all() {
-			return artists;
+		/**
+		 * Creates a new party.
+		 *
+		 * @param id
+		 *            The ID of the party
+		 */
+		public LazyParty(String id) {
+			super(id);
 		}
 
 		//
-		// RESULTPROCESSOR METHODS
+		// PARTY METHODS
 		//
 
 		/**
 		 * {@inheritDoc}
 		 */
-		public void processResult(ResultSet resultSet) throws SQLException {
-			String artistId = resultSet.getString("ARTISTS.ID");
-			if ((artist == null) || (!artist.id().equals(artistId))) {
-				artist = new DefaultArtist(artistId).name(resultSet.getString("ARTISTS.NAME")).groups(new HashSet<Group>());
-				artists.add(artist);
+		@Override
+		public Collection<Track> getReleases() {
+			if (!hasValue("releases")) {
+				try {
+					getValue("releases", Collection.class).set(getTracksByParty(getId()));
+				} catch (DatabaseException de1) {
+					throw new RuntimeException("Could not loaded Tracks for Party " + getId() + ".", de1);
+				}
+			}
+			return super.getReleases();
+		}
+
+	}
+
+	/**
+	 * {@link ObjectCreator} implementation that can create {@link Party}
+	 * objects.
+	 *
+	 * @author <a href="mailto:bombe@pterodactylus.net">David ‘Bombe’ Roden</a>
+	 */
+	private class PartyCreator implements ObjectCreator<Party> {
+
+		/**
+		 * {@inheritDoc}
+		 */
+		@Override
+		public Party createObject(ResultSet resultSet) throws SQLException {
+			return new LazyParty(resultSet.getString("PARTIES.ID")).setName(resultSet.getString("PARTIES.NAME"));
+		}
+
+	}
+
+	/**
+	 * {@link User} implementation that retrieves some attributes (such as
+	 * {@link #getOpenIds()}) from the {@link DataManager} on demand.
+	 *
+	 * @author <a href="mailto:bombe@pterodactylus.net">David ‘Bombe’ Roden</a>
+	 */
+	private class LazyUser extends DefaultUser {
+
+		/** Memoizer for a user’s OpenIDs. */
+		private final Memoizer<Void> openIdMemoizer = new Memoizer<Void>(new Callable<Void>() {
+
+			@Override
+			public Void call() throws Exception {
+				if (!hasValue("openIds")) {
+					getValue("openIds", Collection.class).set(getOpenIdsByUser(getId()));
+				}
+				return null;
 			}
+		});
+
+		/**
+		 * Creates a new user.
+		 *
+		 * @param id
+		 *            The ID of the user
+		 */
+		public LazyUser(String id) {
+			super(id);
+		}
+
+		/**
+		 * {@inheritDoc}
+		 */
+		@Override
+		public Collection<String> getOpenIds() {
+			openIdMemoizer.get();
+			return super.getOpenIds();
+		}
+
+	}
 
-			Group group = new DefaultGroup(resultSet.getString("GROUPS.ID")).name(resultSet.getString("GROUPS.NAME")).url(resultSet.getString("GROUPS.URL"));
-			artist.groups().add(group);
+	/**
+	 * {@link ObjectCreator} implementation that can create {@link User}
+	 * objects. This specific implementation creates {@link LazyUser} instances.
+	 *
+	 * @author <a href="mailto:bombe@pterodactylus.net">David ‘Bombe’ Roden</a>
+	 */
+	private class UserCreator implements ObjectCreator<User> {
+
+		/**
+		 * {@inheritDoc}
+		 */
+		@Override
+		public User createObject(ResultSet resultSet) throws SQLException {
+			return new LazyUser(resultSet.getString("USERS.ID")).setName(resultSet.getString("USERS.NAME")).setPasswordHash(resultSet.getString("USERS.PASSWORD")).setLevel(resultSet.getInt("USERS.LEVEL"));
 		}
 
 	}