diff -r 6105ee2cceaf -r 947d0f6a6a83 src/main/java/de/uapcore/lightpit/dao/ChildEntityDao.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/main/java/de/uapcore/lightpit/dao/ChildEntityDao.java	Thu Oct 15 13:31:52 2020 +0200
@@ -0,0 +1,116 @@
+/*
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.
+ *
+ * Copyright 2018 Mike Becker. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ *   1. Redistributions of source code must retain the above copyright
+ *      notice, this list of conditions and the following disclaimer.
+ *
+ *   2. Redistributions in binary form must reproduce the above copyright
+ *      notice, this list of conditions and the following disclaimer in the
+ *      documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+package de.uapcore.lightpit.dao;
+
+import java.sql.SQLException;
+import java.util.List;
+
+public interface ChildEntityDao<T, P> {
+
+    /**
+     * Lists all entities being a child of the specified parent.
+     * @param parent the parent
+     * @return the list of child instances
+     * @throws SQLException on any kind of SQL errors
+     */
+    List<T> list(P parent) throws SQLException;
+
+    /**
+     * Finds an entity by its integer ID.
+     * It is not guaranteed that referenced entities are automatically joined.
+     *
+     * @param id the id
+     * @return the enity or null if there is no such entity
+     * @throws SQLException on any kind of SQL errors
+     */
+    T find(int id) throws SQLException;
+
+    /**
+     * Inserts an instance into database.
+     * It is not guaranteed that generated fields will be updated in the instance.
+     *
+     * @param instance the instance to insert
+     * @param parent a reference to the parent
+     * @throws SQLException on any kind of SQL errors
+     */
+    void save(T instance, P parent) throws SQLException;
+
+    /**
+     * Updates an instance in the database.
+     *
+     * @param instance the instance to insert
+     * @return true if an instance has been updated, false if no instance with the specified ID was found
+     * @throws SQLException on any kind of SQL errors
+     */
+    boolean update(T instance) throws SQLException;
+
+    /**
+     * Updates an instance in the database changing the parent.
+     * This operation is not supported by default.
+     *
+     * @param instance the instance to insert
+     * @param newParent a reference to the new parent
+     * @return true if an instance has been updated, false if no instance with the specified ID was found
+     * @throws SQLException on any kind of SQL errors
+     * @see #isChangingParentSupported()
+     */
+    default boolean update(T instance, P newParent) throws SQLException {
+        throw new UnsupportedOperationException();
+    }
+
+    /**
+     * Returns true if changing the parent is supported by this DAO.
+     * This method must return true, if {@link #update(Object, Object)} is implemented.
+     * @return true, if changing the parent is supported
+     */
+    default boolean isChangingParentSupported() {
+        return false;
+    }
+
+    /**
+     * Inserts or updates an instance in the database.
+     * Tries an update first and if that fails, performs a save.
+     * If changing a parent is not supported by this DAO,
+     * specifying an alternate parent for an existing instance has no effect.
+     *
+     * @param instance the instance to insert or update
+     * @param parent a reference to the parent
+     * @throws SQLException on any kind of SQL errors
+     * @see #update(Object)
+     * @see #update(Object, Object)
+     * @see #save(Object, Object)
+     */
+    default void saveOrUpdate(T instance, P parent) throws SQLException {
+        if (isChangingParentSupported()) {
+            if (!update(instance, parent)) save(instance, parent);
+        } else {
+            if (!update(instance)) save(instance, parent);
+        }
+    }
+}