# RelationJoinNode.java

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

    import javax.annotation.Nullable;
    import java.util.HashMap;
    import java.util.IdentityHashMap;
    import java.util.Map;
    import java.util.Set;
    import java.util.function.Function;
    import java.util.function.Supplier;

    import org.codefilarete.reflection.Accessor;
    import org.codefilarete.stalactite.engine.runtime.load.EntityJoinTree.JoinType;
    import org.codefilarete.stalactite.engine.runtime.load.EntityTreeInflater.RelationIdentifier;
    import org.codefilarete.stalactite.engine.runtime.load.EntityTreeInflater.TreeInflationContext;
    import org.codefilarete.stalactite.mapping.RowTransformer;
    import org.codefilarete.stalactite.query.model.Fromable;
    import org.codefilarete.stalactite.query.model.JoinLink;
    import org.codefilarete.stalactite.query.model.Selectable;
    import org.codefilarete.stalactite.sql.ddl.structure.Key;
    import org.codefilarete.stalactite.sql.result.BeanRelationFixer;
    import org.codefilarete.stalactite.sql.result.ColumnedRow;
    import org.codefilarete.tool.Reflections;
    import org.codefilarete.tool.VisibleForTesting;
    import org.codefilarete.tool.bean.Objects;

    /**
     * Join node for filling a relation between beans such as one-to-one or one-to-many
     *
     * @author Guillaume Mary
     */
    public class RelationJoinNode<C, T1 extends Fromable, T2 extends Fromable, JOINTYPE, I> extends AbstractJoinNode<C, T1, T2, JOINTYPE> {

    	/** The right part of the join */
    	private final EntityInflater<C, I> entityInflater;

    	private final Accessor<?, ?> propertyAccessor;

    	/** Relation fixer for instances of this strategy on owning strategy entities */
    	private final BeanRelationFixer<Object, C> beanRelationFixer;

    	/** Available only in List cases : gives the identifier of an entity in the List to avoid duplicate mix (typically : concatenates list index to entity id)*/
    	private final Function<ColumnedRow, ?> relationIdentifierProvider;

    	RelationJoinNode(JoinNode<?, T1> parent,
    					 Accessor<?, C> propertyAccessor,
    					 JoinLink<T1, JOINTYPE> leftJoinColumn,
    					 JoinLink<T2, JOINTYPE> rightJoinColumn,
    					 JoinType joinType,
    					 Set<? extends Selectable<?>> columnsToSelect,    // Of T2
    					 @Nullable String tableAlias,
    					 EntityInflater<C, I> entityInflater,
    					 BeanRelationFixer<?, C> beanRelationFixer,
    					 @Nullable Function<ColumnedRow, ?> relationIdentifierProvider) {
    		super(parent, leftJoinColumn, rightJoinColumn, joinType, columnsToSelect, tableAlias);
    		this.entityInflater = entityInflater;
    		this.propertyAccessor = propertyAccessor;
    		this.beanRelationFixer = (BeanRelationFixer<Object, C>) beanRelationFixer;
    		this.relationIdentifierProvider = relationIdentifierProvider;
    	}

    	@VisibleForTesting
    	public RelationJoinNode(JoinNode<?, T1> parent,
    							Accessor<?, ?> propertyAccessor,
    							Key<T1, JOINTYPE> leftJoinColumn,
    							Key<T2, JOINTYPE> rightJoinColumn,
    							JoinType joinType,
    							Set<? extends Selectable<?>> columnsToSelect,    // Of T2
    							@Nullable String tableAlias,
    							EntityInflater<C, I> entityInflater,
    							BeanRelationFixer<?, C> beanRelationFixer,
    							@Nullable Function<ColumnedRow, ?> relationIdentifierProvider) {
    		super(parent, leftJoinColumn, rightJoinColumn, joinType, columnsToSelect, tableAlias);
    		this.entityInflater = entityInflater;
    		this.propertyAccessor = propertyAccessor;
    		this.beanRelationFixer = (BeanRelationFixer<Object, C>) beanRelationFixer;
    		this.relationIdentifierProvider = relationIdentifierProvider;
    	}

    	RelationJoinNode(JoinNode<?, T1> parent,
    					 Accessor<?, ?> propertyAccessor,
    					 Key<T1, JOINTYPE> leftJoinColumn,
    					 Key<T2, JOINTYPE> rightJoinColumn,
    					 JoinType joinType,
    					 Set<? extends Selectable<?>> columnsToSelect,    // Of T2
    					 @Nullable String tableAlias,
    					 EntityInflater<C, I> entityInflater,
    					 BeanRelationFixer<?, C> beanRelationFixer,
    					 @Nullable Function<ColumnedRow, ?> relationIdentifierProvider,
    					 IdentityHashMap<JoinLink<?, ?>, JoinLink<?, ?>> columnClones) {
    		super(parent, leftJoinColumn, rightJoinColumn, joinType, columnsToSelect, tableAlias, columnClones);
    		this.entityInflater = entityInflater;
    		this.propertyAccessor = propertyAccessor;
    		this.beanRelationFixer = (BeanRelationFixer<Object, C>) beanRelationFixer;
    		this.relationIdentifierProvider = relationIdentifierProvider;
    	}

    	public EntityInflater<C, ?> getEntityInflater() {
    		return entityInflater;
    	}

    	public Accessor<?, ?> getPropertyAccessor() {
    		return propertyAccessor;
    	}

    	public Class<C> getEntityType() {
    		return entityInflater.getEntityType();
    	}

    	BeanRelationFixer<Object, C> getBeanRelationFixer() {
    		return beanRelationFixer;
    	}

    	public Function<ColumnedRow, ?> getRelationIdentifierProvider() {
    		return relationIdentifierProvider;
    	}

    	@Override
    	public RelationJoinRowConsumer<C, I> toConsumer(JoinNode<C, T2> joinNode) {
    		return new DefaultRelationJoinRowConsumer<>(joinNode, entityInflater, beanRelationFixer, relationIdentifierProvider, getConsumptionListener());
    	}

    	public interface RelationJoinRowConsumer<C, I> extends JoinRowConsumer {

    		C applyRelatedEntity(Object parentJoinEntity, ColumnedRow row, TreeInflationContext context);
    	}

    	static class DefaultRelationJoinRowConsumer<C, I> implements RelationJoinRowConsumer<C, I> {

    		private final JoinNode<C, ?> joinNode;

    		private final Class<C> entityType;

    		private final Function<ColumnedRow, I> identifierProvider;

    		/** Relation fixer for instances of this strategy on owning strategy entities */
    		private final BeanRelationFixer<Object, C> beanRelationFixer;

    		private final Function<ColumnedRow, ?> relationIdentifierComputer;

    		private final RowTransformer<C> rowTransformer;

    		/** Optional listener of ResultSet decoding */
    		@Nullable
    		private final EntityTreeJoinNodeConsumptionListener<C> consumptionListener;

    		DefaultRelationJoinRowConsumer(JoinNode<C, ?> joinNode,
    									   EntityInflater<C, I> entityInflater,
    									   BeanRelationFixer<Object, C> beanRelationFixer,
    									   @Nullable Function<ColumnedRow, ?> relationIdentifierComputer,
    									   @Nullable EntityTreeJoinNodeConsumptionListener<C> consumptionListener) {
    			this.joinNode = joinNode;
    			this.entityType = entityInflater.getEntityType();
    			this.identifierProvider = entityInflater::giveIdentifier;
    			this.beanRelationFixer = beanRelationFixer;
    			this.relationIdentifierComputer = Objects.preventNull(relationIdentifierComputer, this.identifierProvider);
    			this.rowTransformer = entityInflater.getRowTransformer();
    			this.consumptionListener = consumptionListener;
    		}

    		@Override
    		public JoinNode<C, ?> getNode() {
    			return joinNode;
    		}

    		RowTransformer<C> getRowTransformer() {
    			return rowTransformer;
    		}

    		@Override
    		public C applyRelatedEntity(Object parentJoinEntity, ColumnedRow row, TreeInflationContext context) {
    			I rightIdentifier = identifierProvider.apply(row);
    			// we avoid treating twice same relation, overall to avoid adding twice same instance to a collection (one-to-many list cases)
    			// in case of multiple collections in ResultSet because it creates similar data (through cross join) which are treated as many as
    			// collections cross with each other. This also works for one-to-one relations but produces no bugs. It can also be seen as a performance
    			// enhancement even if it hasn't been measured.
    			RelationIdentifier eventuallyApplied = new RelationIdentifier(parentJoinEntity, this.entityType, relationIdentifierComputer.apply(row), this);
    			// primary key null means no entity => nothing to do
    			if (rightIdentifier != null) {
    				C rightEntity = context.giveEntityFromCache(entityType, rightIdentifier, () -> rowTransformer.transform(row));
    				if (context.isTreatedOrAppend(eventuallyApplied)) {
    					beanRelationFixer.apply(parentJoinEntity, rightEntity);
    					if (this.consumptionListener != null) {
    						this.consumptionListener.onNodeConsumption(rightEntity, row);
    					}
    				}
    				// we return the entity found for the row to let caller go deeper in the hierarchy
    				return rightEntity;
    			}
    			// null is a marker for caller to not go deeper in the hierarchy : no entity was found on row, we can't go deeper
    			return null;
    		}

    		/**
    		 * Implemented for debug. DO NOT RELY ON IT for anything else.
    		 */
    		@Override
    		public String toString() {
    			return Reflections.toString(this.getClass());
    		}
    	}

    	@FunctionalInterface
    	public interface EntityCache {

    		/**
    		 * Expected to retrieve an entity by its class and identifier from cache or instantiates it and put it into the cache
    		 *
    		 * @param clazz the type of the entity
    		 * @param identifier the identifier of the entity (Long, String, ...)
    		 * @param factory the "method" that will be called to create the entity when the entity is not in the cache
    		 * @return the existing instance in the cache or a new object
    		 */
    		<C> C computeIfAbsent(Class<C> clazz, Object identifier, Supplier<C> factory);

    	}

    	/**
    	 * Simple class to ease access or creation to entity from the cache
    	 * @see #computeIfAbsent(Class, Object, Supplier)
    	 */
    	static final class BasicEntityCache implements EntityCache {

    		private final Map<Class, Map<Object, Object>> entityCache = new HashMap<>();

    		public <C> C computeIfAbsent(Class<C> clazz, Object identifier, Supplier<C> factory) {
    			Map<Object, Object> classInstanceCacheByIdentifier = entityCache.computeIfAbsent(clazz, k -> new HashMap<>());
    			return (C) classInstanceCacheByIdentifier.computeIfAbsent(identifier, k -> factory.get());
    		}
    	}
    }