diff --git a/app/src/Lib/Traits/TreeTrait.php b/app/src/Lib/Traits/TreeTrait.php
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+<?php
+/**
+ * COmanage Registry Tree Trait
+ *
+ * Portions licensed to the University Corporation for Advanced Internet
+ * Development, Inc. ("UCAID") under one or more contributor license agreements.
+ * See the NOTICE file distributed with this work for additional information
+ * regarding copyright ownership.
+ *
+ * UCAID licenses this file to you under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with the
+ * License. You may obtain a copy of the License at:
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * @link          https://www.internet2.edu/comanage COmanage Project
+ * @package       registry
+ * @since         COmanage Registry v5.2.0
+ * @license       Apache License, Version 2.0 (http://www.apache.org/licenses/LICENSE-2.0)
+ */
+
+declare(strict_types = 1);
+
+namespace App\Lib\Traits;
+
+use \Cake\Utility\Inflector;
+use \App\Lib\Util\StringUtilities;
+
+// Utility functions for models that use TreeBehavior
+trait TreeTrait {
+  /**
+   * Assemble the set of potential parents. This function is primarily intended for
+   * UI purposes, ie to build a list of available parent entities. In general, potentially
+   * invalid parents are also included, to allow an error message to be thrown when one
+   * is selected.
+   *
+   * @since  COmanage Registry v5.2.0
+   * @param  int    $coId      CO ID
+   * @param  int    $id        Entity ID to determine potential parents of, or null for any (or a new) entity
+   * @param  bool   $hierarchy Render the hierarchy in the name
+   * @param  array  $where     Additional conditions for filtering potential parents
+   * @return array             Array of Entity IDs and Names
+   */
+  
+  public function potentialParents(
+    int $coId, 
+    ?int $id=null, 
+    bool $hierarchy=false,
+    array $where=[]
+  ) {
+    // Note prior to v5 we filtered child COUs, meaning a COU couldn't be reassigned
+    // to be a child of a current child. It's not clear why we imposed that restriction.
+    // We could generate a list of only valid parents by using BETWEEN, see TreeBehavior
+    // _setParents(), eg SELECT * FROM table WHERE lft > $entity->lft AND lft < $entity->rght
+    // or something like that.
+
+    $query = null;
+    
+    if($hierarchy) {
+      $query = $this->find('treeList', spacer: '+ ');
+    } else {
+      $query = $this->find('list');
+    }
+    
+    $query = $query->where(['co_id' => $coId])
+                  // true overrides the default Cake order for treeList so we get
+                  // our items sorted alphabetically instead of by tree ID
+                   ->orderBy(['name' => 'ASC'], true);
+    
+    if($id) {
+      $query = $query->where(['id <>' => $id]);
+    }
+
+    if(!empty($where)) {
+      $query = $query->where($where);
+    }
+    
+    return $query->toArray();
+  }
+
+  /**
+   * Application Rule to determine if the parent ID is a potential parent.
+   * Cake's TreeBehavior will perform checks as part of callbacks (eg) beforeSave,
+   * but the error messages are not localized and fairly opaque. We'll check for
+   * common failure modes here and throw better errors before TreeBehavior does
+   * its thing.
+   *
+   * @since  COmanage Registyr v5.2.0
+   * @param  Entity  $entity  Entity to be validated
+   * @param  array   $options Application rule options
+   * @return boolean          true if the Rule check passes, false otherwise
+   */
+  
+  public function rulePotentialParent($entity, $options) {
+    // The Entity we're working with, for error messages
+    $entityNane = Inflector::singularize(StringUtilities::entityToClassName($entity));
+
+    if(!empty($entity->parent_id)) {
+      // First check that we're not trying to set ourself to be our own parent
+      if($entity->id == $entity->parent_id) {
+        return __d('error', 'tree.parent.same', [$entityNane]);
+      }
+
+      // This check is based on TreeBehavior::_setParent()
+      $parent = $this->get($entity->parent_id);
+
+      if($parent->lft > $entity->lft && $parent->lft < $entity->rght) {
+        return __d('error', 'tree.parent.invalid', [$entityNane]);
+      }
+    }
+
+    return true;
+  }
+}