# IndexedMappedManyRelationDescriptor.java

    package org.codefilarete.stalactite.engine.runtime.onetomany;

    import java.util.Collection;
    import java.util.Comparator;
    import java.util.HashMap;
    import java.util.LinkedList;
    import java.util.List;
    import java.util.Map;
    import java.util.Set;
    import java.util.function.Function;
    import java.util.function.Supplier;
    import javax.annotation.Nullable;

    import org.codefilarete.reflection.PropertyMutator;
    import org.codefilarete.reflection.ReadWritePropertyAccessPoint;
    import org.codefilarete.stalactite.sql.ddl.structure.Column;
    import org.codefilarete.stalactite.sql.ddl.structure.Key;
    import org.codefilarete.stalactite.sql.ddl.structure.Table;
    import org.codefilarete.stalactite.sql.result.BeanRelationFixer;

    import static org.codefilarete.tool.bean.Objects.preventNull;

    /**
     * Container to store information of an indexed *-to-many indexed mapped relation (by a column on the reverse side)
     *
     * @author Guillaume Mary
     */
    public class IndexedMappedManyRelationDescriptor<SRC, TRGT, C extends Collection<TRGT>, SRCID, TRGTID> extends MappedManyRelationDescriptor<SRC, TRGT, C, SRCID> {

    	/** Column that stores index value, owned by reverse side table (table of targetPersister) */
    	private final Column<Table, Integer> indexingColumn;

    	public IndexedMappedManyRelationDescriptor(ReadWritePropertyAccessPoint<SRC, C> collectionGetter,
    											   Supplier<C> collectionFactory,
    											   @Nullable PropertyMutator<TRGT, SRC> reverseSetter,
    											   Key<?, SRCID> reverseColumn,
    											   Column<? extends Table, Integer> indexingColumn,
    											   Function<SRC, SRCID> idProvider,
    											   Function<TRGT, TRGTID> targetIdProvider) {
    		super(collectionGetter, collectionFactory, reverseSetter, reverseColumn);
    		this.indexingColumn = (Column<Table, Integer>) indexingColumn;
    		super.relationFixer = new InMemoryRelationHolder(idProvider, targetIdProvider);
    	}

    	public Column<Table, Integer> getIndexingColumn() {
    		return indexingColumn;
    	}

    	/**
    	 * A relation fixer that doesn't set the Collection onto the source bean but keeps it in memory.
    	 * Made for collection with persisted order: it is sorted later (by some other code) by getting the collection
    	 * with {@link #get(Object)}, and then set it onto source bean (by collection setter).
    	 * Collection is kept in memory thanks to a ThreadLocal because its lifetime is expected to be a select execution,
    	 * hence caller is expected to initialize and clean this instance by calling {@link #init()} and {@link #clear()}
    	 * before and after selection.
    	 *
    	 * @see #applySort(Set)
    	 * @author Guillaume Mary
    	 */
    	public class InMemoryRelationHolder implements BeanRelationFixer<SRC, TRGT> {

    		// Note that we prefer to store Map<SRCID> instead of IdentityMap for now because it's simpler but requires identifier providers.
    		// Could be replaced by IdentityMap<SRC>
    		private final ThreadLocal<Map<SRCID, List<TRGT>>> relationCollectionPerEntity = new ThreadLocal<>();

    		/**
    		 * Context for indexed mapped List. Will keep bean index during select between "unrelated" methods/phases :
    		 * index column must be added to SQL select, read from ResultSet and order applied to sort final List, but this particular feature crosses over
    		 * layers (entities and SQL) which is not implemented. In such circumstances, ThreadLocal comes to the rescue.
    		 * Could be static, but would lack the TRGTID typing, which leads to some generics errors, so left non-static (acceptable small overhead)
    		 */
    		private final ThreadLocal<Map<TRGTID, Integer>> currentSelectedIndexes = new ThreadLocal<>();

    		private final Function<SRC, SRCID> idProvider;
    		private final Function<TRGT, TRGTID> targetIdProvider;

    		public InMemoryRelationHolder(Function<SRC, SRCID> idProvider,
    									  Function<TRGT, TRGTID> targetIdProvider) {
    			this.idProvider = idProvider;
    			this.targetIdProvider = targetIdProvider;
    		}

    		public Map<TRGTID, Integer> getCurrentSelectedIndexes() {
    			return currentSelectedIndexes.get();
    		}

    		@Override
    		public void apply(SRC source, TRGT input) {
    			Map<SRCID, List<TRGT>> srcidcMap = relationCollectionPerEntity.get();
    			List<TRGT> collection = srcidcMap.computeIfAbsent(idProvider.apply(source), id -> new LinkedList<>());
    			collection.add(input);
    			// bidirectional assignment
    			preventNull(getReverseSetter(), NOOP_REVERSE_SETTER).set(input, source);
    		}

    		private List<TRGT> get(SRC src) {
    			Map<SRCID, List<TRGT>> currentMap = relationCollectionPerEntity.get();
    			return currentMap == null ? null : currentMap.get(idProvider.apply(src));
    		}

    		public void init() {
    			this.relationCollectionPerEntity.set(new HashMap<>());
    			this.currentSelectedIndexes.set(new HashMap<>());
    		}

    		public void clear() {
    			this.relationCollectionPerEntity.remove();
    			this.currentSelectedIndexes.remove();
    		}

    		/**
    		 * Reconciliate {@link Collection} order.
    		 * To be called after selecting entities from the database.
    		 *
    		 * @param result the entities to be sorted
    		 */
    		public void applySort(Set<? extends SRC> result) {
    			result.forEach(src -> {
    				List<TRGT> inMemoryCollection = get(src);
    				if (inMemoryCollection != null) {	// inMemoryCollection can be null if there's no associated entity in the database
    					Map<TRGTID, Integer> indexPerTargetId = currentSelectedIndexes.get();
    					inMemoryCollection.sort(Comparator.comparingInt(target -> indexPerTargetId.get(targetIdProvider.apply(target))));
    					C relationCollection = getCollectionAccessPoint().get(src);
    					if (relationCollection == null) {
    						relationCollection = getCollectionFactory().get();
    						getCollectionAccessPoint().set(src, relationCollection);
    					}
    					relationCollection.addAll(inMemoryCollection);
    				}
    			});
    		}
    	}
    }