diff --git a/library/Opus/Db2/Collections.php b/library/Opus/Db2/Collections.php
new file mode 100644
index 00000000..46ef645a
--- /dev/null
+++ b/library/Opus/Db2/Collections.php
@@ -0,0 +1,85 @@
+<?php
+
+/**
+ * This file is part of OPUS. The software OPUS has been originally developed
+ * at the University of Stuttgart with funding from the German Research Net,
+ * the Federal Department of Higher Education and Research and the Ministry
+ * of Science, Research and the Arts of the State of Baden-Wuerttemberg.
+ *
+ * OPUS 4 is a complete rewrite of the original OPUS software and was developed
+ * by the Stuttgart University Library, the Library Service Center
+ * Baden-Wuerttemberg, the Cooperative Library Network Berlin-Brandenburg,
+ * the Saarland University and State Library, the Saxon State Library -
+ * Dresden State and University Library, the Bielefeld University Library and
+ * the University Library of Hamburg University of Technology with funding from
+ * the German Research Foundation and the European Regional Development Fund.
+ *
+ * LICENCE
+ * OPUS is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 2 of the Licence, or any later version.
+ * OPUS is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
+ * details. You should have received a copy of the GNU General Public License
+ * along with OPUS; if not, write to the Free Software Foundation, Inc., 51
+ * Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * @copyright   Copyright (c) 2010-2018, OPUS 4 development team
+ * @license     http://www.gnu.org/licenses/gpl.html General Public License
+ */
+
+namespace Opus\Db2;
+
+/**
+ * Table gateway class to table 'collections'.
+ */
+
+class Collections extends NestedSet
+{
+    /**
+     * Table name of the nested set table.
+     *
+     * @var string
+     */
+    protected $name = 'collections';
+
+    /**
+     * Table column holding the primary key for the nested set structure.
+     *
+     * @var string
+     */
+    protected $primary = 'id';
+
+    /**
+     * Table column holding the left-id for the nested set structure.
+     *
+     * @var string
+     */
+    protected $left = 'left_id';
+
+    /**
+     * Table column holding the right-id for the nested set structure.
+     *
+     * @var string
+     */
+    protected $right = 'right_id';
+
+    /**
+     * Table column holding the parent-id for the structure.  This actually is
+     * more than a nested set structure, but we need this for fast retrieval of
+     * one nodes' children.
+     *
+     * @var string
+     */
+    protected $parent = 'parent_id';
+
+    /**
+     * Table column holding the tree-id for the structure.  We're holding more
+     * than one nested-set structure in the table and we're distinguishing the
+     * different trees by this ID.
+     *
+     * @var string
+     */
+    protected $tree = 'role_id';
+}
diff --git a/library/Opus/Db2/NestedSet.php b/library/Opus/Db2/NestedSet.php
new file mode 100644
index 00000000..dabb7b75
--- /dev/null
+++ b/library/Opus/Db2/NestedSet.php
@@ -0,0 +1,152 @@
+<?php
+
+/**
+ * This file is part of OPUS. The software OPUS has been originally developed
+ * at the University of Stuttgart with funding from the German Research Net,
+ * the Federal Department of Higher Education and Research and the Ministry
+ * of Science, Research and the Arts of the State of Baden-Wuerttemberg.
+ *
+ * OPUS 4 is a complete rewrite of the original OPUS software and was developed
+ * by the Stuttgart University Library, the Library Service Center
+ * Baden-Wuerttemberg, the Cooperative Library Network Berlin-Brandenburg,
+ * the Saarland University and State Library, the Saxon State Library -
+ * Dresden State and University Library, the Bielefeld University Library and
+ * the University Library of Hamburg University of Technology with funding from
+ * the German Research Foundation and the European Regional Development Fund.
+ *
+ * LICENCE
+ * OPUS is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 2 of the Licence, or any later version.
+ * OPUS is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
+ * details. You should have received a copy of the GNU General Public License
+ * along with OPUS; if not, write to the Free Software Foundation, Inc., 51
+ * Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * @copyright   Copyright (c) 2020, OPUS 4 development team
+ * @license     http://www.gnu.org/licenses/gpl.html General Public License
+ */
+
+namespace Opus\Db2;
+
+use Doctrine\DBAL\Query\QueryBuilder;
+use Opus\Model\ModelException;
+
+/**
+ * Table gateway class to nested sets.
+ *
+ * WARNING: This class does not use transactions.  If you want to be transaction
+ * WARNING: safe, beginTransaction() before using methods from here and commit()
+ * WARNING: when you're done.
+ *
+ * ANOTHER WARNING: Always make sure, $treeId, $id and all the parameters you
+ * ANOTHER WARNING: are clean.  Currently, we assume that tree_id and id are
+ * ANOTHER WARNING: integer datatypes and explicitly cast to int.  This might
+ * ANOTHER WARNING: result in strange behaviour, if you're not using integers
+ * ANOTHER WARNING: or submitting NULL values.
+ *
+ * TODO Reevaluate above warnings
+ *
+ * phpcs:disable
+ */
+abstract class NestedSet extends AbstractTableGateway
+{
+    /**
+     * Returns the row data for the node of the given ID.
+     *
+     * @param  int $id Primary key of the node.
+     * @throws ModelException
+     * @return array
+     */
+    public function getNodeById($id)
+    {
+        $select = $this->selectNodeById($id);
+        $row    = $select->execute()->fetchAssociative();
+
+        if ($row === null) {
+            throw new ModelException("Node $id not found.");
+        }
+
+        return $row;
+    }
+
+    /**
+     * Returns an SQL query builder instance preconfigured with an SQL statement
+     * for retrieving nodes by ID.
+     *
+     * @param  int $id Primary key of the node.
+     * @return QueryBuilder
+     */
+    private function selectNodeById($id)
+    {
+        $queryBuilder = $this->getQueryBuilder();
+
+        $select = $queryBuilder
+            ->select('*')
+            ->from($this->name, 'node')
+            ->where("node.{$this->primary} = ?");
+
+        return $select->setParameters([$id]);
+    }
+
+    /**
+     * Returns the row data for all child nodes of the node with given ID.
+     *
+     * @param  int   $id The ID of the node whose children shall be returned.
+     * @return array
+     */
+    public function getChildrenById($id)
+    {
+        $select = $this->selectChildrenById($id, 'id');
+
+        return $select->execute()->fetchAllAssociative();
+    }
+
+    /**
+     * Returns an SQL query builder instance preconfigured with an SQL statement
+     * for fetching all children of the node with the given ID. A second
+     * parameter allows the selection of the returned columns in the same format
+     * as \QueryBuilder::select() takes.
+     *
+     * @param int   $id   The ID of the parent node.
+     * @param mixed $cols The columns to show, defaults to '*'.
+     *
+     * @return QueryBuilder
+     */
+    public function selectChildrenById($id, $cols = '*')
+    {
+        $queryBuilder = $this->getQueryBuilder();
+
+        $select = $queryBuilder
+            ->select($cols)
+            ->from($this->name, 'node')
+            ->where("node.{$this->parent} = ?")
+            ->orderBy("node.{$this->left}", "ASC");
+
+        return $select->setParameters([$id]);
+    }
+
+    /**
+     * Returns the row data for all nodes with the given tree ID. Returns null
+     * if nothing was found.
+     *
+     * @param  int   $treeId The ID of the nested-set structure whose nodes shall be returned.
+     * @return array|null
+     */
+    public function getNodesByTreeId($treeId)
+    {
+        $queryBuilder = $this->getQueryBuilder();
+
+        $select = $queryBuilder
+            ->select('*')
+            ->from($this->name)
+            ->where("{$this->tree} = ?")
+            ->orderBy("{$this->left}", "ASC");
+
+        $select->setParameters([$treeId]);
+
+        return $queryBuilder->execute()->fetchAllAssociative();
+    }
+}
diff --git a/tests/Opus/CollectionTest.php b/tests/Opus/CollectionTest.php
index 62c3d736..d548b7c7 100644
--- a/tests/Opus/CollectionTest.php
+++ b/tests/Opus/CollectionTest.php
@@ -43,7 +43,7 @@
 use Opus\Collection;
 use Opus\CollectionRole;
 use Opus\Config;
-use Opus\Db\Collections;
+use Opus\Db2\Collections;
 use Opus\Document;
 use Opus\Model\NotFoundException;
 use Opus\Model\Xml\Cache;
@@ -1235,14 +1235,12 @@ protected function validateNestedSet()
     {
         $table = new Collections();
 
-        $select = $table->select()->where('role_id = ?', 1)->order('left_id ASC');
-
-        $rows = $table->fetchAll($select);
+        $rows = $table->getNodesByTreeId(1);
 
         $this->assertEquals(14, count($rows));
 
-        $this->assertEquals(1, $rows[0]->left_id);
-        $this->assertEquals(28, $rows[0]->right_id);
+        $this->assertEquals(1, $rows[0]["left_id"]);
+        $this->assertEquals(28, $rows[0]["right_id"]);
 
         $validator = new NestedSetValidator($table);
 
diff --git a/tests/Opus/TestAsset/NestedSetValidator.php b/tests/Opus/TestAsset/NestedSetValidator.php
index 9221f0cb..066cd372 100644
--- a/tests/Opus/TestAsset/NestedSetValidator.php
+++ b/tests/Opus/TestAsset/NestedSetValidator.php
@@ -35,7 +35,7 @@
 
 namespace OpusTest\TestAsset;
 
-use Opus\Db\NestedSet;
+use Opus\Db2\NestedSet;
 use Opus\LoggingTrait;
 use Opus\Model\ModelException;
 
@@ -81,8 +81,7 @@ public function __construct($table)
      */
     public function validate($rootId)
     {
-        $select        = $this->table->select()->where('id = ?', $rootId);
-        $node          = $this->table->fetchRow($select);
+        $node          = $this->table->getNodeById($rootId);
         $this->counter = (int) $node['left_id'];
         return $this->validateNode($rootId); // root node
     }
@@ -97,8 +96,7 @@ public function validateNode($nodeId)
     {
         $logger = $this->getLogger();
 
-        $select  = $this->table->select()->where('id = ?', $nodeId);
-        $node    = $this->table->fetchRow($select);
+        $node    = $this->table->getNodeById($nodeId);
         $leftId  = $node['left_id'];
         $rightId = $node['right_id'];
 
@@ -121,11 +119,7 @@ public function validateNode($nodeId)
             // node
             if ($distance & 1) {
                 // odd; valid
-                $selectChildren = $this->table->select()->where(
-                    'parent_id  = ?',
-                    $nodeId
-                )->order('left_id ASC');
-                 $children      = $this->table->fetchAll($selectChildren);
+                $children = $this->table->getChildrenById($nodeId);
                 foreach ($children as $child) {
                     if ($this->validateNode($child['id']) === false) {
                         return false;