# BeforeDeleteByIdCollectionCascader.java

    package org.codefilarete.stalactite.engine.cascade;

    import java.util.Collection;
    import java.util.stream.Collectors;

    import org.codefilarete.tool.collection.Iterables;
    import org.codefilarete.stalactite.engine.EntityPersister;
    import org.codefilarete.stalactite.engine.listener.DeleteByIdListener;
    import org.codefilarete.stalactite.engine.listener.PersisterListenerCollection;

    /**
     * Cascader for delete, written for one-to-many style of cascade where Target owns the relation to Trigger
     *
     * @param <TRIGGER> type of the elements that trigger this collection cascade
     * @param <TARGET> collection elements type
     * @author Guillaume Mary
     */
    public abstract class BeforeDeleteByIdCollectionCascader<TRIGGER, TARGET> implements DeleteByIdListener<TRIGGER> {

    	private final EntityPersister<TARGET, ?> persister;

    	/**
    	 * Simple constructor. Created instance must be added to {@link PersisterListenerCollection} afterward.
    	 * @param persister
    	 */
    	public BeforeDeleteByIdCollectionCascader(EntityPersister<TARGET, ?> persister) {
    		this.persister = persister;
    		this.persister.addDeleteByIdListener(new DeleteByIdListener<TARGET>() {
    			@Override
    			public void afterDeleteById(Iterable<? extends TARGET> entities) {
    				postTargetDelete((Iterable<TARGET>) entities);
    			}
    		});
    	}

    	public EntityPersister<TARGET, ?> getPersister() {
    		return persister;
    	}

    	/**
    	 * Overridden to delete Target instances of the Trigger instances.
    	 *
    	 * @param entities source entities previously deleted
    	 */
    	@Override
    	public void beforeDeleteById(Iterable<? extends TRIGGER> entities) {
    		this.persister.deleteById(Iterables.stream(entities).flatMap(c -> getTargets(c).stream()).collect(Collectors.toList()));
    	}

    	/**
    	 * Post treatment after Target instance deletion. Cache cleanup for instance.
    	 *
    	 * @param entities entities deleted by this listener
    	 */
    	protected abstract void postTargetDelete(Iterable<TARGET> entities);

    	/**
    	 * Expected to give the Target instances of a Trigger (should simply give a field value of trigger)
    	 *
    	 * @param trigger the source instance from which to take the targets
    	 * @return the linked objets or null if there's not (or shouldn't be persisted for whatever reason)
    	 */
    	protected abstract Collection<TARGET> getTargets(TRIGGER trigger);

    }