# TablePerClassPolymorphismPersister.java

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

    import javax.annotation.Nullable;
    import java.util.Collection;
    import java.util.Collections;
    import java.util.HashMap;
    import java.util.HashSet;
    import java.util.Map;
    import java.util.Queue;
    import java.util.Set;
    import java.util.function.BiFunction;
    import java.util.function.Function;
    import java.util.stream.Collectors;

    import org.codefilarete.reflection.Accessor;
    import org.codefilarete.stalactite.engine.DeleteExecutor;
    import org.codefilarete.stalactite.engine.EntityPersister;
    import org.codefilarete.stalactite.engine.InsertExecutor;
    import org.codefilarete.stalactite.engine.SelectExecutor;
    import org.codefilarete.stalactite.engine.UpdateExecutor;
    import org.codefilarete.stalactite.engine.configurer.builder.PersisterBuilderContext;
    import org.codefilarete.stalactite.engine.configurer.onetomany.OneToManyRelationConfigurer;
    import org.codefilarete.stalactite.engine.runtime.AbstractPolymorphismPersister;
    import org.codefilarete.stalactite.engine.runtime.ConfiguredRelationalPersister;
    import org.codefilarete.stalactite.engine.runtime.EntityMappingWrapper;
    import org.codefilarete.stalactite.engine.runtime.FirstPhaseRelationLoader;
    import org.codefilarete.stalactite.engine.runtime.PersisterWrapper;
    import org.codefilarete.stalactite.engine.runtime.PolymorphicPersister;
    import org.codefilarete.stalactite.engine.runtime.RelationIds;
    import org.codefilarete.stalactite.engine.runtime.RelationalEntityPersister;
    import org.codefilarete.stalactite.engine.runtime.SecondPhaseRelationLoader;
    import org.codefilarete.stalactite.engine.runtime.load.EntityInflater.EntityMappingAdapter;
    import org.codefilarete.stalactite.engine.runtime.load.EntityJoinTree;
    import org.codefilarete.stalactite.engine.runtime.load.EntityJoinTree.JoinType;
    import org.codefilarete.stalactite.engine.runtime.load.EntityMerger.EntityMergerAdapter;
    import org.codefilarete.stalactite.engine.runtime.load.JoinNode;
    import org.codefilarete.stalactite.engine.runtime.load.MergeJoinNode;
    import org.codefilarete.stalactite.engine.runtime.load.PolymorphicMergeJoinRowConsumer;
    import org.codefilarete.stalactite.engine.runtime.load.TablePerClassPolymorphicRelationJoinNode;
    import org.codefilarete.stalactite.mapping.EntityMapping;
    import org.codefilarete.stalactite.mapping.IdMapping;
    import org.codefilarete.stalactite.mapping.Mapping.ShadowColumnValueProvider;
    import org.codefilarete.stalactite.mapping.RowTransformer.TransformerListener;
    import org.codefilarete.stalactite.query.api.Fromable;
    import org.codefilarete.stalactite.query.api.JoinLink;
    import org.codefilarete.stalactite.query.model.Query;
    import org.codefilarete.stalactite.query.api.QueryStatement.PseudoColumn;
    import org.codefilarete.stalactite.query.api.QueryStatement.PseudoTable;
    import org.codefilarete.stalactite.query.api.Selectable;
    import org.codefilarete.stalactite.query.api.Selectable.SimpleSelectable;
    import org.codefilarete.stalactite.query.model.Union;
    import org.codefilarete.stalactite.sql.ConnectionProvider;
    import org.codefilarete.stalactite.sql.Dialect;
    import org.codefilarete.stalactite.sql.ddl.structure.Column;
    import org.codefilarete.stalactite.sql.ddl.structure.Key;
    import org.codefilarete.stalactite.sql.ddl.structure.Key.KeyBuilder;
    import org.codefilarete.stalactite.sql.ddl.structure.PrimaryKey;
    import org.codefilarete.stalactite.sql.ddl.structure.Table;
    import org.codefilarete.stalactite.sql.result.BeanRelationFixer;
    import org.codefilarete.stalactite.sql.result.ColumnedRow;
    import org.codefilarete.tool.Duo;
    import org.codefilarete.tool.bean.Objects;
    import org.codefilarete.tool.collection.Iterables;
    import org.codefilarete.tool.collection.KeepOrderMap;
    import org.codefilarete.tool.collection.KeepOrderSet;
    import org.codefilarete.tool.collection.Maps;
    import org.codefilarete.tool.function.Hanger.Holder;
    import org.codefilarete.tool.trace.MutableInt;

    import static org.codefilarete.stalactite.engine.runtime.load.EntityJoinTree.ROOT_JOIN_NAME;
    import static org.codefilarete.stalactite.query.Operators.cast;

    /**
     * @author Guillaume Mary
     */
    public class TablePerClassPolymorphismPersister<C, I, T extends Table<T>> extends AbstractPolymorphismPersister<C, I> {

    	@SuppressWarnings("java:S5164" /* remove() is called by SecondPhaseRelationLoader.afterSelect() */)
    	private static final ThreadLocal<Queue<Set<RelationIds<Object /* E */, Object /* target */, Object /* target identifier */>>>> DIFFERED_ENTITY_LOADER = new ThreadLocal<>();

    	public TablePerClassPolymorphismPersister(ConfiguredRelationalPersister<C, I> mainPersister,
    											  Map<? extends Class<C>, ? extends ConfiguredRelationalPersister<C, I>> subEntitiesPersisters,
    											  ConnectionProvider connectionProvider,
    											  Dialect dialect) {
    		super(mainPersister,
    				subEntitiesPersisters,
    				new TablePerClassPolymorphismEntityFinder<C, I, T>(
    						mainPersister,
    						subEntitiesPersisters,
    						connectionProvider,
    						dialect));

    		this.subEntitiesPersisters.forEach((type, persister) ->
    				mainPersister.getEntityJoinTree().projectTo(persister.getEntityJoinTree(), ROOT_JOIN_NAME)
    		);

    	}

    	@Override
    	public <LEFTTABLE extends Table<LEFTTABLE>, SUBTABLE extends Table<SUBTABLE>, JOINTYPE> void propagateMappedAssociationToSubTables(
    			Key<SUBTABLE, JOINTYPE> foreignKey,
    			PrimaryKey<LEFTTABLE, JOINTYPE> leftPrimaryKey, BiFunction<Key<SUBTABLE, JOINTYPE>, PrimaryKey<LEFTTABLE, JOINTYPE>, String> foreignKeyNamingFunction) {
    		subEntitiesPersisters.values().stream().forEach(subPersister -> {
    			SUBTABLE subTable = subPersister.getMainTable();
    			KeyBuilder<SUBTABLE, JOINTYPE> projectedKeyBuilder = Key.from(subTable);
    			((Set<Column<SUBTABLE, ?>>) foreignKey.getColumns()).forEach(column -> {
    				Column<SUBTABLE, ?> subtableColumn = subTable.addColumn(column.getName(), column.getJavaType(), column.getSize(), column.isNullable());
    				projectedKeyBuilder.addColumn(subtableColumn);
    				subPersister.getEntityJoinTree().getRoot().getOriginalColumnsToLocalOnes().put(subtableColumn, subtableColumn);
    			});
    			Key<SUBTABLE, JOINTYPE> projectedKey = projectedKeyBuilder.build();
    			subPersister.getEntityJoinTree().addPassiveJoin(ROOT_JOIN_NAME, foreignKey, projectedKey, JoinType.INNER, Collections.emptySet());
    			subTable.addForeignKey(foreignKeyNamingFunction, projectedKey, leftPrimaryKey);
    		});
    	}

    	@Override
    	public Set<Class<? extends C>> getSupportedEntityTypes() {
    		// Note that this implementation can't be tested yet because we don't yet support table-per-class polymorphism
    		// combined with other polymorphism types. It has only been implemented in order to not forget this behavior for the day of the polymorphism
    		// combination has come
    		Set<Class<? extends C>> result = new HashSet<>();
    		this.subEntitiesPersisters.forEach((c, p) -> {
    			if (p instanceof PolymorphicPersister) {
    				result.addAll((Collection) ((PolymorphicPersister<?>) p).getSupportedEntityTypes());
    			} else if (p instanceof PersisterWrapper && ((PersisterWrapper<C, I>) p).getDeepestDelegate() instanceof PolymorphicPersister) {
    				result.addAll(((PolymorphicPersister) ((PersisterWrapper) p).getDeepestDelegate()).getSupportedEntityTypes());
    			} else {
    				result.add(c);
    			}
    		});
    		return result;
    	}

    	@Override
    	public Collection<Table<?>> giveImpliedTables() {
    		// in table-per-class main persister table does not participate in database schema : only sub entities persisters do
    		return subEntitiesPersisters.values().stream().flatMap(p -> p.giveImpliedTables().stream()).collect(Collectors.toList());
    	}

    	@Override
    	public void doInsert(Iterable<? extends C> entities) {
    		Map<EntityPersister<C, I>, Set<C>> entitiesPerType = computeEntitiesPerPersister(entities);
    		entitiesPerType.forEach(InsertExecutor::insert);
    	}

    	@Override
    	public void doUpdateById(Iterable<? extends C> entities) {
    		Map<EntityPersister<C, I>, Set<C>> entitiesPerType = computeEntitiesPerPersister(entities);
    		entitiesPerType.forEach(UpdateExecutor::updateById);
    	}

    	@Override
    	public void doUpdate(Iterable<? extends Duo<C, C>> differencesIterable, boolean allColumnsStatement) {
    		// Below we keep the order of given entities mainly to get steady unit tests. Meanwhile, this may have performance
    		// impacts but it's very difficult to measure
    		Map<UpdateExecutor<C>, Set<Duo<C, C>>> entitiesPerType = new KeepOrderMap<>();
    		differencesIterable.forEach(payload ->
    				this.subEntitiesPersisters.values().forEach(persister -> {
    					C entity = Objects.preventNull(payload.getLeft(), payload.getRight());
    					if (persister.getClassToPersist().isInstance(entity)) {
    						entitiesPerType.computeIfAbsent(persister, p -> new KeepOrderSet<>()).add(payload);
    					}
    				})
    		);

    		entitiesPerType.forEach((updateExecutor, adhocEntities) -> updateExecutor.update(adhocEntities, allColumnsStatement));
    	}

    	@Override
    	public void doDelete(Iterable<? extends C> entities) {
    		Map<EntityPersister<C, I>, Set<C>> entitiesPerType = computeEntitiesPerPersister(entities);
    		entitiesPerType.forEach(DeleteExecutor::delete);
    	}

    	@Override
    	public void doDeleteById(Iterable<? extends C> entities) {
    		Map<EntityPersister<C, I>, Set<C>> entitiesPerType = computeEntitiesPerPersister(entities);
    		entitiesPerType.forEach(DeleteExecutor::deleteById);
    	}

    	private <D extends C> Map<EntityPersister<D, I>, Set<D>> computeEntitiesPerPersister(Iterable<? extends C> entities) {
    		// Below we keep the order of given entities mainly to get steady unit tests. Meanwhile, this may have performance
    		// impacts but it's very difficult to measure
    		Map<EntityPersister<D, I>, Set<D>> entitiesPerType = new KeepOrderMap<>();
    		entities.forEach(entity ->
    				this.subEntitiesPersisters.values().forEach(persister -> {
    					if (persister.getClassToPersist().isInstance(entity)) {
    						entitiesPerType.computeIfAbsent((EntityPersister<D, I>) persister, p -> new KeepOrderSet<>()).add((D) entity);
    					}
    				})
    		);
    		return entitiesPerType;
    	}

    	@Override
    	public <E, ID> void copyRootJoinsTo(EntityJoinTree<E, ID> entityJoinTree, String joinName) {
    		getEntityJoinTree().projectTo(getEntityJoinTree(), joinName);
    	}

    	/**
    	 * Overridden to capture {@link EntityMapping#addShadowColumnInsert(ShadowColumnValueProvider)} and
    	 * {@link EntityMapping#addShadowColumnUpdate(ShadowColumnValueProvider)} (see {@link OneToManyRelationConfigurer})
    	 * Made to dispatch those methods subclass strategies since their persisters are in charge of managing their entities (not the parent one).
    	 * <p>
    	 * Design question : one may think that's not a good design to override a getter, caller should invoke an intention-clear method on
    	 * ourselves (Persister) but the case is to add a silent Column insert/update which is not the goal of the Persister to know implementation
    	 * detail : they are to manage cascades and coordinate their mapping strategies. {@link EntityMapping} are in charge of knowing
    	 * {@link Column} actions.
    	 *
    	 * @return an enhanced version of our main persister mapping strategy which dispatches silent column insert/update to sub-entities ones
    	 */
    	@Override
    	public EntityMapping<C, I, T> getMapping() {
    		return new EntityMappingWrapper<C, I, T>(mainPersister.getMapping()) {
    			@Override
    			public void addTransformerListener(TransformerListener<C> listener) {
    				Collection<? extends ConfiguredRelationalPersister<C, I>> subPersisters = subEntitiesPersisters.values();
    				subPersisters.forEach(persister -> persister.getMapping().addTransformerListener(listener));
    			}

    			@Override
    			public void addShadowColumnInsert(ShadowColumnValueProvider<C, T> provider) {
    				subEntitiesPersisters.values().forEach(p -> {
    					p.<Table>getMapping().addShadowColumnInsert(projectShadowColumnProvider(provider, p));
    				});
    			}

    			@Override
    			public void addShadowColumnUpdate(ShadowColumnValueProvider<C, T> provider) {
    				subEntitiesPersisters.values().forEach(p -> {
    					p.<Table>getMapping().addShadowColumnUpdate(projectShadowColumnProvider(provider, p));
    				});
    			}

    			private <D extends C, SUBENTITYTABLE extends Table<SUBENTITYTABLE>>
    			ShadowColumnValueProvider<D, SUBENTITYTABLE>
    			projectShadowColumnProvider(ShadowColumnValueProvider<C, T> provider, ConfiguredRelationalPersister<D, I> subEntityPersister) {

    				Map<Column<T, ?>, Column<SUBENTITYTABLE, ?>> projectedColumnMap = new HashMap<>(provider.getColumns().size());
    				provider.getColumns().forEach(c -> {
    					Column<SUBENTITYTABLE, ?> projectedColumn = subEntityPersister.<SUBENTITYTABLE>getMapping().getTargetTable().addColumn(c.getName(), c.getJavaType(), c.getSize(), c.isNullable());
    					projectedColumnMap.put(c, projectedColumn);
    				});
    				return new ShadowColumnValueProvider<D, SUBENTITYTABLE>() {

    					private final Set<Column<SUBENTITYTABLE, ?>> values = new HashSet<>(projectedColumnMap.values());

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

    					@Override
    					public Map<Column<SUBENTITYTABLE, ?>, ?> giveValue(D bean) {
    						Map<Column<T, ?>, ?> columnObjectMap = provider.giveValue(bean);
    						return Maps.innerJoin(projectedColumnMap, columnObjectMap);
    					}
    				};
    			}
    		};
    	}

    	@Override
    	public <SRC, T1 extends Table<T1>, T2 extends Table<T2>, SRCID, JOINID> String joinAsOne(RelationalEntityPersister<SRC, SRCID> sourcePersister,
    																							 Accessor<SRC, C> propertyAccessor,
    																							 Key<T1, JOINID> leftColumn,
    																							 Key<T2, JOINID> rightColumn,
    																							 String rightTableAlias,
    																							 BeanRelationFixer<SRC, C> beanRelationFixer,
    																							 boolean optional,
    																							 boolean loadSeparately) {
    		if (loadSeparately) {
    			String createdJoinNodeName = this.joinAsOneWithSeparateLoading(sourcePersister.getEntityJoinTree(), ROOT_JOIN_NAME,
    					leftColumn,
    					rightColumn,
    					(Set<? extends ConfiguredRelationalPersister<C, I>>) (Set) new HashSet<>(this.subEntitiesPersisters.values()));

    			sourcePersister.addSelectListener(new SecondPhaseRelationLoader<>(beanRelationFixer, DIFFERED_ENTITY_LOADER));

    			return createdJoinNodeName;
    		} else {
    			return join(
    					sourcePersister.getEntityJoinTree(),
                        ROOT_JOIN_NAME,
    					propertyAccessor,
    					leftColumn,
    					rightColumn,
    					beanRelationFixer);
    		}
    	}

    	@Override
    	public <SRC, T1 extends Table<T1>, T2 extends Table<T2>, SRCID, JOINID> String joinAsMany(String joinName,
    																							  RelationalEntityPersister<SRC, SRCID> sourcePersister,
    																							  Accessor<SRC, ?> propertyAccessor,
    																							  Key<T1, JOINID> leftColumn,
    																							  Key<T2, JOINID> rightColumn,
    																							  BeanRelationFixer<SRC, C> beanRelationFixer,
    																							  @Nullable Function<ColumnedRow, Object> duplicateIdentifierProvider,
    																							  Set<? extends Column<T2, ?>> selectableColumns,
    																							  boolean optional,
    																							  boolean loadSeparately) {

    		PrimaryKey<T, ?> mainTablePK = mainPersister.<T>getMapping().getTargetTable().getPrimaryKey();
    		Map<ConfiguredRelationalPersister, Key> joinColumnPerSubPersister = new HashMap<>();
    		if (rightColumn.equals(mainTablePK)) {
    			// join is made on primary key => case is association table
    			subEntitiesPersisters.forEach((c, subPersister) -> {
    				joinColumnPerSubPersister.put(subPersister, subPersister.getMapping().getTargetTable().getPrimaryKey());
    			});
    		} else {
    			// join is made on a foreign key => case of relation owned by reverse side
    			subEntitiesPersisters.forEach((c, subPersister) -> {
    				KeyBuilder<?, Object> reverseKey = projectPrimaryKey(rightColumn, subPersister);
    				joinColumnPerSubPersister.put(subPersister, reverseKey.build());
    			});
    		}

    		if (loadSeparately) {
    			// TODO: simplify query : it joins on target table as many as subentities which can be reduced to one join if FirstPhaseRelationLoader
    			//  can compute discriminatorValue
    			subEntitiesPersisters.forEach((c, subPersister) -> {
    				sourcePersister.getEntityJoinTree().addMergeJoin(
    						joinName,
    						// need to be cast to C instead of "? extends C" because selectExecutor is C-typed
    						new FirstPhaseRelationLoader<>(subPersister.getMapping().getIdMapping(), this, (ThreadLocal) DIFFERED_ENTITY_LOADER),
    						leftColumn,
    						joinColumnPerSubPersister.get(subPersister),
    						JoinType.OUTER);
    			});

    			// adding second phase loader
    			sourcePersister.addSelectListener(new SecondPhaseRelationLoader<>(beanRelationFixer, DIFFERED_ENTITY_LOADER));

    			// FIXME : we shouldn't return null here but a created join node name: which one since we have several table to join ? see joinAsOne(..) maybe ?
    			return null;
    		} else {
    			return join(
    					sourcePersister.getEntityJoinTree(),
    					joinName,
    					propertyAccessor,
    					leftColumn,
    					rightColumn,
    					beanRelationFixer);
    		}
    	}

    	private <MAINTABLE extends Table<MAINTABLE>, SUBTABLE extends Table<SUBTABLE>, JOINID> KeyBuilder<SUBTABLE, Object>
    	projectPrimaryKey(Key<MAINTABLE, JOINID> rightColumn, ConfiguredRelationalPersister<? extends C, I> subPersister) {
    		EntityMapping<? extends C, I, SUBTABLE> subTypeMapping = subPersister.getMapping();
    		KeyBuilder<SUBTABLE, Object> reverseKey = Key.from(subTypeMapping.getTargetTable());
    		rightColumn.getColumns().forEach(col -> {
    			Column<SUBTABLE, ?> column = subTypeMapping.getTargetTable().addColumn(col.getExpression(), col.getJavaType());
    			subTypeMapping.addShadowColumnSelect(column);
    			reverseKey.addColumn(column);
    		});
    		return reverseKey;
    	}

    	private <SRC, SRCID, T1 extends Table<T1>, T2 extends Table<T2>, JOINCOLTYPE> String join(
    			EntityJoinTree<SRC, SRCID> entityJoinTree,
    			String leftStrategyName,
    			Accessor<SRC, ?> propertyAccessor,
    			Key<T1, JOINCOLTYPE> leftJoinColumn,
    			Key<T2, JOINCOLTYPE> rightJoinColumn,
    			BeanRelationFixer<SRC, C> beanRelationFixer) {

    		// we build a union of all sub queries that will be joined in the main query
    		// To build the union we need columns that are common to all persisters
    		Set<JoinLink<?, ?>> commonColumns = new KeepOrderSet<>();
    		commonColumns.addAll(mainPersister.getMapping().getSelectableColumns());
    		// TODO : right column is not in selected columns of class mapping : understand why (and if that's normal)
    		commonColumns.addAll(rightJoinColumn.getColumns());

    		Set<String> commonColumnsNames = commonColumns.stream().map(JoinLink::getExpression).collect(Collectors.toSet());

    		Set<ConfiguredRelationalPersister<? extends C, I>> subPersisters = new HashSet<>(this.subEntitiesPersisters.values());

    		KeepOrderSet<Column<?, ?>> nonCommonColumns = new KeepOrderSet<>();
    		subPersisters.forEach(subPersister -> {
    			nonCommonColumns.addAll(subPersister.getMainTable().getColumns());
    		});
    		nonCommonColumns.removeIf(c -> commonColumnsNames.contains(c.getName()));

    		Union subPersistersUnion = new Union();
    		String entityTypeDiscriminatorName = "clazz_";
    		PseudoColumn<Integer> discriminatorPseudoColumn = subPersistersUnion.registerColumn(entityTypeDiscriminatorName, Integer.class);
    		MutableInt discriminatorComputer = new MutableInt();

    		subPersisters.forEach(subPersister -> {
    			Query subEntityQuery = new Query(subPersister.getMapping().getTargetTable());
    			subEntityQuery.select(String.valueOf(discriminatorComputer.increment()), Integer.class, entityTypeDiscriminatorName);
    			subPersistersUnion.unionAll(subEntityQuery);

    			commonColumns.forEach(column -> {
    				subEntityQuery.select(column.getExpression(), column.getJavaType());
    				subPersistersUnion.registerColumn(column.getExpression(), column.getJavaType());
    			});

    			nonCommonColumns.forEach(column -> {
    				Selectable<?> expression;
    				if (subPersister.getMapping().getSelectableColumns().contains(column)) {
    					expression = new SimpleSelectable<>(column.getName(), column.getJavaType());
    				} else {
    					expression = cast((String) null, column.getJavaType());
    				}
    				// we put an alias else cast(..) as no name which makes it doesn't match official-column name, and then
    				// may cause an error since SQL in kind of invalid
    				subEntityQuery.select(expression, column.getName());
    				subPersistersUnion.registerColumn(column.getName(), column.getJavaType());
    			});
    		});

    		Holder<TablePerClassPolymorphicRelationJoinNode<C, T1, JOINCOLTYPE, I>> createdJoinHolder = new Holder<>();
    		String relationJoinName = entityJoinTree.addJoin(leftStrategyName, parent -> {
    			TablePerClassPolymorphicRelationJoinNode<C, T1, JOINCOLTYPE, I> relationJoinNode = new TablePerClassPolymorphicRelationJoinNode<>(
    					(JoinNode<SRC, T1>) (JoinNode) parent,
    					subPersistersUnion,
    					propertyAccessor,
    					leftJoinColumn,
    					rightJoinColumn,
    					JoinType.OUTER,
    					subPersistersUnion.getColumns(),
    					mainPersister.getClassToPersist().getSimpleName(),
    					new EntityMappingAdapter<>(mainPersister.<T1>getMapping()),
    					(BeanRelationFixer<Object, C>) beanRelationFixer,
    					discriminatorPseudoColumn);
    			createdJoinHolder.set(relationJoinNode);
    			return relationJoinNode;
    		});

    		this.addTablePerClassPolymorphicSubPersistersJoins(entityJoinTree, relationJoinName, createdJoinHolder.get(), subPersisters);

    		return relationJoinName;
    	}

    	private <SRC, SRCID, V extends C, T1 extends Table<T1>, T2 extends Table<T2>> void addTablePerClassPolymorphicSubPersistersJoins(
    			EntityJoinTree<SRC, SRCID> entityJoinTree,
    			String mainPolymorphicJoinNodeName,
    			TablePerClassPolymorphicRelationJoinNode<C, T1, ?, I> mainPersisterJoin,
    			Set<ConfiguredRelationalPersister<? extends C, I>> subPersisters) {

    		// The join is made on the Union as left table, and the column we must get is mainPersister's primaryKey (which table is not in the tree since
    		// we are the table-per-class case), so we have to create an equivalent of the primary key, based on the columns of the union
    		KeyBuilder<PseudoTable, I> leftKey = Key.from(mainPersisterJoin.getRightTable());
    		mainPersister.<T1>getMainTable().getPrimaryKey().getColumns().forEach(pkCol -> {
    			JoinLink<PseudoTable, ?> selectable = (JoinLink<PseudoTable, ?>) Iterables.find(mainPersisterJoin.getColumnsToSelect(), selectableColumn -> selectableColumn.getExpression().equals(pkCol.getName()));
    			leftKey.addColumn(selectable);
    		});
    		MutableInt discriminatorComputer = new MutableInt();
    		subPersisters.forEach(subPersister -> {
    			ConfiguredRelationalPersister<V, I> localSubPersister = (ConfiguredRelationalPersister<V, I>) subPersister;
    			entityJoinTree.addMergeJoin(mainPolymorphicJoinNodeName,
    					new EntityMergerAdapter<>(localSubPersister.<T2>getMapping()),
    					leftKey.build(),
    					subPersister.<T2>getMainTable().getPrimaryKey(),
    					JoinType.OUTER,
    					joinNode -> {
    						PolymorphicMergeJoinRowConsumer<V, I> joinRowConsumer = new PolymorphicMergeJoinRowConsumer<>(
    								(MergeJoinNode) joinNode,
    								localSubPersister.<T2>getMapping());
    						mainPersisterJoin.addSubPersisterJoin(joinRowConsumer, discriminatorComputer.increment());
    						return joinRowConsumer;
    					});
    		});
    	}

    	/**
    	 * Makes a join between source entity tree and a lite union of all sub-persisters table in order to get target
    	 * entities ids in a first query. Ids will be kept in a {@link ThreadLocal} to fully load target entities in
    	 * a separate query (see caller).
    	 */
    	private <SRC, T1 extends Table<T1>, T2 extends Table<T2>, SRCID, JOINID> String joinAsOneWithSeparateLoading(
    			EntityJoinTree<SRC, SRCID> entityJoinTree,
    			String leftStrategyName,
    			Key<T1, JOINID> leftJoinColumn,
    			Key<T2, JOINID> rightJoinColumn,
    			Set<? extends ConfiguredRelationalPersister<C, I>> subPersisters) {

    		Union subPersistersUnion = new Union();
    		// Union will contain only 3 columns :
    		// - discriminator
    		// - entity primary key
    		// - join column
    		String entityTypeDiscriminatorName = "clazz_";
    		subPersistersUnion.registerColumn(entityTypeDiscriminatorName, Integer.class);

    		// adding the pseudo columns for the primary key of the main entity to create the entity identifier
    		PrimaryKey<T, I> primaryKey = mainPersister.<T>getMainTable().getPrimaryKey();
    		primaryKey.getColumns().forEach(column -> {
    			subPersistersUnion.registerColumn(column.getExpression(), column.getJavaType(), column.getExpression());
    		});

    		MutableInt discriminatorComputer = new MutableInt();
    		Map<Integer, SelectExecutor<? extends C, I>> subtypeSelectorPerDiscriminatorValue = new HashMap<>();

    		subPersisters.forEach(subPersister -> {
    			Query subEntityQuery = new Query(subPersister.getMapping().getTargetTable());
    			int discriminatorValue = discriminatorComputer.increment();
    			subtypeSelectorPerDiscriminatorValue.put(discriminatorValue, subPersister);
    			subEntityQuery.select(String.valueOf(discriminatorValue), Integer.class, entityTypeDiscriminatorName);
    			subPersistersUnion.unionAll(subEntityQuery);

    			rightJoinColumn.getColumns().forEach(column -> {
    				subEntityQuery.select(column.getExpression(), column.getJavaType());
    			});

    			// we add sub primary key columns to make them available to the union, then they can be used to create the entity identifier
    			// through idMapping.getIdentifierAssembler().getColumns()
    			primaryKey.getColumns().forEach(column -> {
    				subEntityQuery.select(column.getName(), column.getJavaType());
    			});
    		});

    		// adding the join columns as being selectable in the union
    		rightJoinColumn.getColumns().forEach(column -> {
    			subPersistersUnion.registerColumn(column.getExpression(), column.getJavaType(), column.getExpression());
    		});

    		PseudoTable pseudoTable = subPersistersUnion.asPseudoTable("unioned_" + mainPersister.getClassToPersist().getSimpleName());
    		PseudoColumn<Integer> discriminatorPseudoColumn = pseudoTable.findColumn(entityTypeDiscriminatorName);
    		Set<PseudoColumn<?>> primaryKeyPseudoColumns = new KeepOrderSet<>();
    		primaryKey.getColumns().forEach(column -> {
    			primaryKeyPseudoColumns.add(pseudoTable.findColumn(column.getName()));
    		});
    		TablePerClassFirstPhaseRelationLoader tablePerClassFirstPhaseRelationLoader = new TablePerClassFirstPhaseRelationLoader(
    				mainPersister.getMapping().getIdMapping(),
    				this,
    				(ThreadLocal<Queue<Set<RelationIds<Object, C, I>>>>) (ThreadLocal) DIFFERED_ENTITY_LOADER,
    				subtypeSelectorPerDiscriminatorValue,
    				discriminatorPseudoColumn,
    				primaryKeyPseudoColumns
    		);

    		// We take right join column from "union all" query : actually it must be taken from union made as a pseudo table
    		// because taking it directly from Union is incorrect since a Union doesn't implement Fromable (because it hasn't
    		// any alias)
    		KeyBuilder<Fromable, JOINID> rightJoinLinkBuilder = Key.from(pseudoTable);
    		rightJoinColumn.getColumns().forEach(column -> {
    			rightJoinLinkBuilder.addColumn(pseudoTable.findColumn(column.getExpression()));
    		});

    		String createdJoinName = entityJoinTree.addMergeJoin(leftStrategyName,
    				tablePerClassFirstPhaseRelationLoader,
    				leftJoinColumn,
    				rightJoinLinkBuilder.build(),
    				JoinType.OUTER);

    		// Because sub-entities are part of the union and not in the tree as a join node, there are not seen as some DDL participant,
    		// hence we add them to it. That's a bit ugly, but I didn't find a better way to do it.
    		subPersisters.forEach(subPersister -> {
    			EntityJoinTree<C, I> subEntityJoinTree = subPersister.getEntityJoinTree();
    			subEntityJoinTree.giveTables().forEach(table -> {
    				PersisterBuilderContext.CURRENT.get();
    				entityJoinTree.addTableToIncludeToDDL(table);
    			});
    		});

    		return createdJoinName;
    	}

    	private class TablePerClassFirstPhaseRelationLoader extends FirstPhaseRelationLoader<C, I> {

    		private final PseudoColumn<Integer> discriminatorColumn;
    		private final Map<Integer, SelectExecutor<? extends C, I>> subtypeSelectorPerDiscriminatorValue;
    		private final Set<PseudoColumn<?>> primaryKeyPseudoColumns;

    		private TablePerClassFirstPhaseRelationLoader(IdMapping<C, I> subEntityIdMapping,
    													  SelectExecutor<C, I> selectExecutor,
    													  ThreadLocal<Queue<Set<RelationIds<Object, C, I>>>> relationIdsHolder,
    													  Map<Integer, SelectExecutor<? extends C, I>> subtypeSelectorPerDiscriminatorValue,
    													  PseudoColumn<Integer> discriminatorColumn,
    													  Set<PseudoColumn<?>> primaryKeyPseudoColumns) {
    			// Note that selectExecutor won't be used because we dynamically lookup for it in fillCurrentRelationIds
    			super(subEntityIdMapping, selectExecutor, relationIdsHolder);
    			this.discriminatorColumn = discriminatorColumn;
    			this.subtypeSelectorPerDiscriminatorValue = subtypeSelectorPerDiscriminatorValue;
    			this.primaryKeyPseudoColumns = primaryKeyPseudoColumns;
    		}

    		@Override
    		protected void fillCurrentRelationIds(ColumnedRow columnedRow, C bean) {
    			Integer discriminator = columnedRow.get(discriminatorColumn);
    			// we avoid NPE on polymorphismPolicy.getClass(discriminator) caused by null discriminator in case of empty relation
    			// by only treating known discriminator values (preferred way to check against null because type can be primitive)
    			SelectExecutor<? extends C, I> subSelectExecutor = subtypeSelectorPerDiscriminatorValue.get(discriminator);
    			if (subSelectExecutor != null) {
    				I id = idMapping.getIdentifierAssembler().assemble(columnedRow);
    				addToCurrentIdsHolder(bean, subSelectExecutor, id);
    			}
    		}

    		private <D extends C> void addToCurrentIdsHolder(C bean, SelectExecutor<D, I> subSelectExecutor, I id) {
    			Set<RelationIds<Object, C, I>> relationIds = relationIdsHolder.get().peek();
    			RelationIds<C, D, I> e = new RelationIds<>(subSelectExecutor, idMapping.getIdAccessor()::getId, bean, id);
    			relationIds.add((RelationIds<Object, C, I>) e);
    		}

    		@Override
    		public Set<Selectable<?>> getSelectableColumns() {
    			Set<Selectable<?>> result = new HashSet<>(primaryKeyPseudoColumns);
    //			Set<Selectable<?>> result = new HashSet<>();//idMapping.getIdentifierAssembler().getColumns());
    			result.add(discriminatorColumn);
    			return result;
    		}
    	}
    }