# IndexedAssociationTableManyRelationDescriptor.java

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

    import java.util.Collection;
    import java.util.HashMap;
    import java.util.LinkedList;
    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.Accessor;
    import org.codefilarete.reflection.Mutator;
    import org.codefilarete.stalactite.sql.result.BeanRelationFixer;

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

    /**
     * Container to store information of an indexed *-to-many with association table relation (by a column on the association table)
     *
     * @author Guillaume Mary
     */
    public class IndexedAssociationTableManyRelationDescriptor<SRC, TRGT, C extends Collection<TRGT>, SRCID> extends ManyRelationDescriptor<SRC, TRGT, C> {

    	/**
    	 * @param collectionGetter  collection accessor
    	 * @param collectionSetter  collection setter
    	 * @param collectionFactory collection factory
    	 * @param reverseSetter
    	 */
    	public IndexedAssociationTableManyRelationDescriptor(Accessor<SRC, C> collectionGetter,
    														 Mutator<SRC, C> collectionSetter,
    														 Supplier<C> collectionFactory,
    														 @Nullable Mutator<TRGT, SRC> reverseSetter,
    														 Function<SRC, SRCID> idProvider) {
    		super(collectionGetter, collectionSetter, collectionFactory, reverseSetter);
    		super.relationFixer = new InMemoryRelationHolder(idProvider);
    	}

    	@Override
    	public InMemoryRelationHolder getRelationFixer() {
    		return (InMemoryRelationHolder) super.getRelationFixer();
    	}

    	/**
    	 * 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 with {@link #applySort(Set)} (to be called by some afterSelect(..) code),
    	 * and 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> {

    		private class CollectionOrderStorage {

    			// Note that we use index as key instead of target entities to allow them to appear twice in the same collection (List),
    			// Moreover, thanks to this form, it's easy to be applied to final Collection because values() automatically returns the entities as sorted
    			private final Map<Integer /* index */, TRGT> targetPerIndex = new HashMap<>();
    		}

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

    		private final Function<SRC, SRCID> idProvider;

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

    		public void addIndex(SRCID leftEntityId, TRGT trgt, int index) {
    			currentSelectedIndexes.get().get(leftEntityId).targetPerIndex.put(index, trgt);
    		}

    		@Override
    		public void apply(SRC source, TRGT input) {
    			// we store the relation in memory without setting it onto source entity because we need to sort it later
    			currentSelectedIndexes.get().computeIfAbsent(idProvider.apply(source), srcid -> new CollectionOrderStorage());
    			// bidirectional assignment
    			preventNull(getReverseSetter(), NOOP_REVERSE_SETTER).set(input, source);
    		}

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

    		public void clear() {
    			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 -> {
    				CollectionOrderStorage inMemoryCollection = currentSelectedIndexes.get().get(idProvider.apply(src));
    				if (inMemoryCollection != null) {  // inMemoryCollection can be null if there's no associated entity in the database
    					Map<Integer, TRGT> targetIdPerId = inMemoryCollection.targetPerIndex;
    					C relationCollection = getCollectionGetter().get(src);
    					if (relationCollection == null) {
    						relationCollection = getCollectionFactory().get();
    						getCollectionSetter().set(src, relationCollection);
    					}
    					relationCollection.addAll(new LinkedList<>(targetIdPerId.values()));
    				}
    			});
    		}
    	}
    }