# BeforeInsertCollectionCascader.java

    package org.codefilarete.stalactite.engine.cascade;

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

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

    /**
     * Cascader for insert, 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 BeforeInsertCollectionCascader<TRIGGER, TARGET> implements InsertListener<TRIGGER> {

    	private final EntityPersister<TARGET, ?> persister;

    	/**
    	 * Simple constructor. Created instance must be added to {@link PersisterListenerCollection} afterward.
    	 *
    	 * @param persister
    	 */
    	public BeforeInsertCollectionCascader(EntityPersister<TARGET, ?> persister) {
    		this.persister = persister;
    	}

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

    	/**
    	 * Overridden to insert Target instances of the Trigger instances.
    	 *
    	 * @param entities source entities previously inserted
    	 */
    	@Override
    	public void beforeInsert(Iterable<? extends TRIGGER> entities) {
    		Stream<TRIGGER> stream = Iterables.stream(entities);
    		Stream<TARGET> targetStream = stream.flatMap(c -> getTargets(c).stream());
    		// We collect things in a Set to avoid persisting duplicates twice which may produce constraint exception : the Set is an identity Set to
    		// avoid basing our comparison on implemented equals/hashCode although this could be sufficient, identity seems safer and match our logic.
    		Set<TARGET> collect = targetStream.collect(Collectors.toCollection(Collections::newIdentitySet));
    		this.persister.persist(collect);
    	}

    	/**
    	 * 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);

    }