# ColumnedCollectionMapping.java

    package org.codefilarete.stalactite.mapping;

    import java.util.ArrayList;
    import java.util.Collection;
    import java.util.HashMap;
    import java.util.Iterator;
    import java.util.LinkedHashMap;
    import java.util.LinkedHashSet;
    import java.util.Map;
    import java.util.Set;
    import java.util.function.Function;

    import org.codefilarete.reflection.ReversibleAccessor;
    import org.codefilarete.reflection.ValueAccessPoint;
    import org.codefilarete.stalactite.sql.ddl.structure.Column;
    import org.codefilarete.stalactite.sql.ddl.structure.Table;
    import org.codefilarete.stalactite.sql.result.ColumnedRow;
    import org.codefilarete.stalactite.sql.statement.binder.PreparedStatementWriter;
    import org.codefilarete.stalactite.sql.statement.binder.ResultSetReader;
    import org.codefilarete.tool.Duo;
    import org.codefilarete.tool.Reflections;
    import org.codefilarete.tool.collection.Collections;
    import org.codefilarete.tool.collection.Iterables;
    import org.codefilarete.tool.collection.PairIterator;
    import org.codefilarete.tool.collection.PairIterator.EmptyIterator;
    import org.codefilarete.tool.collection.PairIterator.InfiniteIterator;
    import org.codefilarete.tool.collection.PairIterator.UntilBothIterator;
    import org.codefilarete.tool.function.Predicates;

    /**
     * A class that "roughly" persists a {@link Collection} to some {@link Column}s : {@link Collection} values are written to given {@link Column}s.
     * Write is made in iteration order. One may change this behavior by overriding {@link #toCollectionValue(Object)} and {@link #toDatabaseValue(Object)}.
     *
     * @author Guillaume Mary
     */
    public class ColumnedCollectionMapping<C extends Collection<O>, O, T extends Table<T>> implements EmbeddedBeanMapping<C, T> {

    	private final T targetTable;
    	private final Set<Column<T, ?>> columns;
    	private final ToCollectionRowTransformer<C> rowTransformer;
    	private final Class<C> persistedClass;

    	/**
    	 * Constructor
    	 *
    	 * @param targetTable table to persist in
    	 * @param columns columns that will be used for persistent of Collections, expected to be a subset of targetTable columns
    	 * @param rowClass Class to instantiate for select from database
    	 */
    	public ColumnedCollectionMapping(T targetTable, Set<? extends Column<T, ?>> columns, Class<C> rowClass) {
    		this.targetTable = targetTable;
    		this.columns = (Set<Column<T, ?>>) columns;
    		this.persistedClass = rowClass;
    		this.rowTransformer = new LocalToCollectionRowTransformer<C>(getPersistedClass(), this::toCollectionValue);
    	}

    	public Class<C> getPersistedClass() {
    		return persistedClass;
    	}

    	public T getTargetTable() {
    		return targetTable;
    	}

    	@Override
    	public Set<Column<T, ?>> getColumns() {
    		return columns;
    	}

    	@Override
    	public ToCollectionRowTransformer<C> getRowTransformer() {
    		return rowTransformer;
    	}

    	@Override
    	public void addPropertySetByConstructor(ValueAccessPoint<C> accessor) {
    		// this class doesn't support bean factory so it can't support properties set by constructor
    	}

    	@Override
    	public Map<Column<T, ?>, ?> getInsertValues(C c) {
    		Collection<O> toIterate = c;
    		if (Collections.isEmpty(c)) {
    			toIterate = new ArrayList<>();
    		}
    		// NB: we wrap c.iterator() in an InfiniteIterator to get all columns generated: overflow columns will have
    		// null value (see 	InfiniteIterator#getValue)
    		PairIterator<Column<T, ?>, O> valueColumnPairIterator = new PairIterator<>(columns.iterator(), new InfiniteIterator<>(toIterate.iterator()));
    		return Iterables.map(() -> valueColumnPairIterator, Duo::getLeft, e -> toDatabaseValue(e.getRight()));
    	}

    	@Override
    	public Map<UpwhereColumn<T>, ?> getUpdateValues(C modified, C unmodified, boolean allColumns) {
    		Map<Column<T, ?>, Object> toReturn = new HashMap<>();
    		if (modified != null) {
    			// getting differences side by side
    			Map<Column<T, ?>, O> unmodifiedColumns = new LinkedHashMap<>();
    			Iterator<O> unmodifiedIterator = unmodified == null ? new EmptyIterator<>() : unmodified.iterator();
    			UntilBothIterator<? extends O, ? extends O> untilBothIterator = new UntilBothIterator<>(modified.iterator(), unmodifiedIterator);
    			PairIterator<Column<T, ?>, Duo<? extends O, ? extends O>> valueColumnPairIterator = new PairIterator<>(columns.iterator(), untilBothIterator);
    			valueColumnPairIterator.forEachRemaining(diffEntry -> {
    				Column<T, ?> fieldColumn = diffEntry.getLeft();
    				Duo<? extends O, ? extends O> toBeCompared = diffEntry.getRight();
    				if (!Predicates.equalOrNull(toBeCompared.getLeft(), toBeCompared.getRight())) {
    					toReturn.put(fieldColumn, toDatabaseValue(toBeCompared.getLeft()));
    				} else {
    					unmodifiedColumns.put(fieldColumn, toBeCompared.getRight());
    				}
    			});

    			// adding complementary columns if necessary
    			if (allColumns && !toReturn.isEmpty()) {
    				Set<Column<T, ?>> missingColumns = new LinkedHashSet<>(columns);
    				missingColumns.removeAll(toReturn.keySet());
    				for (Column<T, ?> missingColumn : missingColumns) {
    					Object missingValue = unmodifiedColumns.get(missingColumn);
    					toReturn.put(missingColumn, missingValue);
    				}
    			}
    		} else if (allColumns && unmodified != null) {
    			for (Column<T, ?> column : columns) {
    				toReturn.put(column, null);
    			}
    		}

    		return convertToUpwhereColumn(toReturn);
    	}

    	private Map<UpwhereColumn<T>, ?> convertToUpwhereColumn(Map<? extends Column<T, ?>, ?> map) {
    		Map<UpwhereColumn<T>, Object> convertion = new HashMap<>();
    		map.forEach((c, s) -> convertion.put(new UpwhereColumn<>(c, true), s));
    		return convertion;
    	}

    	/**
    	 * Gives the database (JDBC) value of the argument.
    	 * This implementation returns the given argument without transformation.
    	 * This may duplicate behavior of {@link PreparedStatementWriter} in some way, but is located to this strategy so can be
    	 * more accurate.
    	 *
    	 * @param object any object took from a persistent collection
    	 * @return the value to be persisted
    	 */
    	protected Object toDatabaseValue(O object) {
    		return object;
    	}

    	/**
    	 * Opposit of {@link #toDatabaseValue(Object)}: converts the database value for the collection value
    	 * This implementation returns the given argument without transformation.
    	 * This may duplicate behavior of {@link ResultSetReader} in some way, but is located to this strategy so can be
    	 * more accurate.
    	 *
    	 * @param object the value coming from the database {@link java.sql.ResultSet}
    	 * @return a value for a Map
    	 */
    	protected O toCollectionValue(Object object) {
    		return (O) object;
    	}

    	@Override
    	public C transform(ColumnedRow row) {
    		return this.rowTransformer.transform(row);
    	}

    	@Override
    	public Map<ReversibleAccessor<C, ?>, Column<T, ?>> getPropertyToColumn() {
    		throw new UnsupportedOperationException(Reflections.toString(ColumnedCollectionMapping.class) + " can't export a mapping between some accessors and their columns");
    	}

    	@Override
    	public Map<ReversibleAccessor<C, ?>, Column<T, ?>> getReadonlyPropertyToColumn() {
    		throw new UnsupportedOperationException(Reflections.toString(ColumnedCollectionMapping.class) + " can't export a mapping between some accessors and their columns");
    	}

    	@Override
    	public Set<Column<T, ?>> getWritableColumns() {
    		return this.columns;
    	}

    	@Override
    	public Set<Column<T, ?>> getReadonlyColumns() {
    		return java.util.Collections.emptySet();
    	}

    	private class LocalToCollectionRowTransformer<C extends Collection> extends ToCollectionRowTransformer<C> {

    		private final Function<Object, Object> databaseValueConverter;

    		private LocalToCollectionRowTransformer(Class<C> persistedClass, Function<Object, Object> databaseValueConverter) {
    			super(persistedClass);
    			this.databaseValueConverter = databaseValueConverter;
    		}

    		private LocalToCollectionRowTransformer(Function<ColumnedRow, C> beanFactory,
    												Function<Object, Object> databaseValueConverter) {
    			super(beanFactory);
    			this.databaseValueConverter = databaseValueConverter;
    		}

    		/** We bind conversion on {@link ColumnedCollectionMapping} conversion methods */
    		@Override
    		public void applyRowToBean(ColumnedRow row, C collection) {
    			for (Column<T, ?> column : columns) {
    				Object value = row.get(column);
    				collection.add(this.databaseValueConverter.apply(value));
    			}
    		}
    	}
    }