diff --git a/application/Espo/ORM/Mapper/BaseMapper.php b/application/Espo/ORM/Mapper/BaseMapper.php
index 763d858c3c..aafdbb4175 100644
--- a/application/Espo/ORM/Mapper/BaseMapper.php
+++ b/application/Espo/ORM/Mapper/BaseMapper.php
@@ -53,7 +53,7 @@ use RuntimeException;
 /**
  * Abstraction for DB. Mapping of Entity to DB. Supposed to be used only internally. Use repositories instead.
  */
-class BaseMapper implements Mapper
+class BaseMapper implements RDBMapper
 {
     const ATTRIBUTE_DELETED = 'deleted';
 
@@ -218,7 +218,7 @@ class BaseMapper implements Mapper
     /**
      * Select related entities from DB.
      *
-     * @return ?SthCollection|Entity
+     * @return Collection|Entity|null
      */
     public function selectRelated(Entity $entity, string $relationName, ?Select $select = null)
     {
@@ -441,17 +441,22 @@ class BaseMapper implements Mapper
     /**
      * Relate an entity with another entity.
      */
-    public function relate(Entity $entity, string $relationName, Entity $foreignEntity, ?array $columnData = null): bool
-    {
+    public function relate(
+        Entity $entity,
+        string $relationName,
+        Entity $foreignEntity,
+        ?array $columnData = null
+    ): bool {
+
         return $this->addRelation($entity, $relationName, null, $foreignEntity, $columnData);
     }
 
     /**
      * Unrelate an entity from another entity.
      */
-    public function unrelate(Entity $entity, string $relationName, Entity $foreignEntity): bool
+    public function unrelate(Entity $entity, string $relationName, Entity $foreignEntity): void
     {
-        return $this->removeRelation($entity, $relationName, null, false, $foreignEntity);
+        $this->removeRelation($entity, $relationName, null, false, $foreignEntity);
     }
 
     /**
@@ -465,35 +470,35 @@ class BaseMapper implements Mapper
     /**
      * Unrelate an entity from another entity by a given ID.
      */
-    public function unrelateById(Entity $entity, string $relationName, string $id): bool
+    public function unrelateById(Entity $entity, string $relationName, string $id): void
     {
-        return $this->removeRelation($entity, $relationName, $id);
+        $this->removeRelation($entity, $relationName, $id);
     }
 
     /**
      * Unrelate all related entities.
      */
-    public function unrelateAll(Entity $entity, string $relationName): bool
+    public function unrelateAll(Entity $entity, string $relationName): void
     {
-        return $this->removeRelation($entity, $relationName, null, true);
+        $this->removeRelation($entity, $relationName, null, true);
     }
 
     /**
      * Update relationship columns.
      */
     public function updateRelationColumns(
-        BaseEntity $entity,
+        Entity $entity,
         string $relationName,
-        ?string $id = null,
+        string $id,
         array $columnData
-    ): bool {
+    ): void {
 
         if (empty($id) || empty($relationName)) {
             throw new RuntimeException("Can't update relation, empty ID or relation name.");
         }
 
         if (empty($columnData)) {
-            return false;
+            return;
         }
 
         $keySet = $this->helper->getRelationKeys($entity, $relationName);
@@ -515,7 +520,7 @@ class BaseMapper implements Mapper
                 }
 
                 if (empty($update)) {
-                    return true;
+                    return;
                 }
 
                 $where = [
@@ -540,7 +545,7 @@ class BaseMapper implements Mapper
 
                 $this->executeSql($sql, true);
 
-                return true;
+                return;
         }
 
         throw new LogicException("Relation type '{$relType}' is not supported.");
@@ -624,7 +629,7 @@ class BaseMapper implements Mapper
     /**
      * Mass relate.
      */
-    public function massRelate(BaseEntity $entity, string $relationName, Select $select): void
+    public function massRelate(Entity $entity, string $relationName, Select $select): void
     {
         $params = $select->getRaw();
 
@@ -1010,7 +1015,7 @@ class BaseMapper implements Mapper
         ?string $id = null,
         bool $all = false,
         ?Entity $relEntity = null
-    ): bool {
+    ): void {
 
         if ($relEntity) {
             $id = $relEntity->id;
@@ -1098,7 +1103,7 @@ class BaseMapper implements Mapper
 
                 $this->executeSql($sql, true);
 
-                return true;
+                return;
 
             case Entity::HAS_ONE:
             case Entity::HAS_MANY:
@@ -1135,7 +1140,7 @@ class BaseMapper implements Mapper
 
                 $this->executeSql($sql, true);
 
-                return true;
+                return;
 
             case Entity::MANY_MANY:
                 $nearKey = $keySet['nearKey'];
@@ -1173,7 +1178,7 @@ class BaseMapper implements Mapper
 
                 $this->executeSql($sql, true);
 
-                return true;
+                return;
         }
 
         throw new LogicException("Relation type '{$relType}' is not supported for unrelating.");
diff --git a/application/Espo/ORM/Mapper/RDBMapper.php b/application/Espo/ORM/Mapper/RDBMapper.php
new file mode 100644
index 0000000000..a6b5236b6a
--- /dev/null
+++ b/application/Espo/ORM/Mapper/RDBMapper.php
@@ -0,0 +1,98 @@
+<?php
+/************************************************************************
+ * This file is part of EspoCRM.
+ *
+ * EspoCRM - Open Source CRM application.
+ * Copyright (C) 2014-2021 Yurii Kuznietsov, Taras Machyshyn, Oleksii Avramenko
+ * Website: https://www.espocrm.com
+ *
+ * EspoCRM 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 3 of the License, or
+ * (at your option) any later version.
+ *
+ * EspoCRM 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 EspoCRM. If not, see http://www.gnu.org/licenses/.
+ *
+ * The interactive user interfaces in modified source and object code versions
+ * of this program must display Appropriate Legal Notices, as required under
+ * Section 5 of the GNU General Public License version 3.
+ *
+ * In accordance with Section 7(b) of the GNU General Public License version 3,
+ * these Appropriate Legal Notices must retain the display of the "EspoCRM" word.
+ ************************************************************************/
+
+namespace Espo\ORM\Mapper;
+
+use Espo\ORM\{
+    Entity,
+    Collection,
+    QueryParams\Select,
+};
+
+interface RDBMapper extends Mapper
+{
+    /**
+     * Relate an entity with another entity.
+     */
+    public function relate(Entity $entity, string $relationName, Entity $foreignEntity, ?array $columnData): bool;
+
+    /**
+     * Unrelate an entity from another entity.
+     */
+    public function unrelate(Entity $entity, string $relationName, Entity $foreignEntity): void;
+
+    /**
+     * Unrelate an entity from another entity by a given ID.
+     */
+    public function relateById(Entity $entity, string $relationName, string $id, ?array $columnData = null): bool;
+
+    /**
+     * Unrelate an entity from another entity by a given ID.
+     */
+    public function unrelateById(Entity $entity, string $relationName, string $id): void;
+
+    /**
+     * Mass relate.
+     */
+    public function massRelate(Entity $entity, string $relationName, Select $select): void;
+
+    /**
+     * Update relationship columns.
+     */
+    public function updateRelationColumns(
+        Entity $entity,
+        string $relationName,
+        string $id,
+        array $columnData
+    ): void;
+
+    /**
+     * Get a relationship column value.
+     *
+     * @return string|int|float|bool|null A relationship column value.
+     */
+    public function getRelationColumn(
+        Entity $entity,
+        string $relationName,
+        string $id,
+        string $column
+    );
+
+    /**
+     * Select related entities from DB.
+     *
+     * @return Collection|Entity|null
+     */
+    public function selectRelated(Entity $entity, string $relationName, ?Select $select = null);
+
+    /**
+     * Get a number of related entities in DB.
+     */
+    public function countRelated(Entity $entity, string $relationName, ?Select $select = null): int;
+}
diff --git a/application/Espo/ORM/Repository/RDBRelation.php b/application/Espo/ORM/Repository/RDBRelation.php
index 4a643c4260..2a4a9ea670 100644
--- a/application/Espo/ORM/Repository/RDBRelation.php
+++ b/application/Espo/ORM/Repository/RDBRelation.php
@@ -34,7 +34,7 @@ use Espo\ORM\{
     Entity,
     EntityManager,
     QueryParams\Select,
-    Mapper\Mapper,
+    Mapper\RDBMapper,
     Repository\RDBRelationSelectBuilder as Builder,
 };
 
@@ -124,7 +124,7 @@ class RDBRelation
         return $this->relationType === Entity::BELONGS_TO_PARENT;
     }
 
-    protected function getMapper(): Mapper
+    protected function getMapper(): RDBMapper
     {
         return $this->entityManager->getMapper();
     }
@@ -245,7 +245,7 @@ class RDBRelation
      * @see Espo\ORM\QueryParams\SelectBuilder::order()
      *
      * @param string|int|array $orderBy
-      * @param bool|string $direction
+     * @param bool|string $direction
      */
     public function order($orderBy, $direction = 'ASC'): Builder
     {
@@ -380,6 +380,7 @@ class RDBRelation
         }
 
         $seed = $this->entityManager->getEntityFactory()->create($this->foreignEntityType);
+
         $seed->set('id', $id);
 
         $this->relate($seed, $columnData, $options);
@@ -399,6 +400,7 @@ class RDBRelation
         }
 
         $seed = $this->entityManager->getEntityFactory()->create($this->foreignEntityType);
+
         $seed->set('id', $id);
 
         $this->unrelate($seed, $options);
@@ -450,11 +452,7 @@ class RDBRelation
 
         $this->beforeUnrelate($entity, $options);
 
-        $result = $this->getMapper()->unrelate($this->entity, $this->relationName, $entity);
-
-        if (!$result) {
-            return;
-        }
+        $this->getMapper()->unrelate($this->entity, $this->relationName, $entity);
 
         $this->afterUnrelate($entity, $options);
     }
@@ -487,7 +485,7 @@ class RDBRelation
             throw new RuntimeException("Can't update not many-to-many relation.");
         }
 
-        $this->getMapper()->updateRelationColumns($this->entity, $this->relationName, $entity->id, $columnData);
+        $this->getMapper()->updateRelationColumns($this->entity, $this->relationName, $entity->getId(), $columnData);
     }
 
     /**
@@ -503,7 +501,7 @@ class RDBRelation
             throw new RuntimeException("Can't get a column of not many-to-many relation.");
         }
 
-        return $this->getMapper()->getRelationColumn($this->entity, $this->relationName, $entity->id, $column);
+        return $this->getMapper()->getRelationColumn($this->entity, $this->relationName, $entity->getId(), $column);
     }
 
     protected function beforeRelate(Entity $entity, ?array $columnData, array $options): void
diff --git a/application/Espo/ORM/Repository/RDBRepository.php b/application/Espo/ORM/Repository/RDBRepository.php
index dfd200baad..07ba1214cd 100644
--- a/application/Espo/ORM/Repository/RDBRepository.php
+++ b/application/Espo/ORM/Repository/RDBRepository.php
@@ -35,7 +35,7 @@ use Espo\ORM\{
     Collection,
     SthCollection,
     Entity,
-    Mapper\Mapper,
+    Mapper\RDBMapper,
     QueryParams\Select,
 };
 
@@ -57,18 +57,14 @@ class RDBRepository extends Repository
         EntityFactory $entityFactory,
         ?HookMediator $hookMediator = null
     ) {
-        $this->entityType = $entityType;
-        $this->entityManager = $entityManager;
-        $this->entityFactory = $entityFactory;
-
-        $this->seed = $this->entityFactory->create($entityType);
+        parent::__construct($entityType, $entityManager, $entityFactory);
 
         $this->hookMediator = $hookMediator ?? (new EmptyHookMediator());
 
         $this->transactionManager = new RDBTransactionManager($entityManager->getTransactionManager());
     }
 
-    protected function getMapper(): Mapper
+    protected function getMapper(): RDBMapper
     {
         return $this->entityManager->getMapper();
     }
@@ -345,12 +341,10 @@ class RDBRepository extends Repository
             ];
         }
 
-        $select = $this->entityManager->getQueryBuilder()
+        return $this->entityManager->getQueryBuilder()
             ->clone($select)
             ->select($selectItemList)
             ->build();
-
-        return $select;
     }
 
     protected function applyRelationAdditionalColumnsConditions(
@@ -506,6 +500,7 @@ class RDBRepository extends Repository
         $this->beforeUnrelate($entity, $relationName, $foreign, $options);
 
         $beforeMethodName = 'beforeUnrelate' . ucfirst($relationName);
+
         if (method_exists($this, $beforeMethodName)) {
             $this->$beforeMethodName($entity, $foreign, $options);
         }
@@ -515,23 +510,25 @@ class RDBRepository extends Repository
         $methodName = 'unrelate' . ucfirst($relationName);
 
         if (method_exists($this, $methodName)) {
-            $result = $this->$methodName($entity, $foreign);
-        } else {
+            $this->$methodName($entity, $foreign);
+        }
+        else {
             if ($foreign instanceof Entity) {
-                $result = $this->getMapper()->unrelate($entity, $relationName, $foreign);
-            } else {
+                $this->getMapper()->unrelate($entity, $relationName, $foreign);
+            }
+            else {
                 $id = $foreign;
-                $result = $this->getMapper()->unrelateById($entity, $relationName, $id);
+
+                $this->getMapper()->unrelateById($entity, $relationName, $id);
             }
         }
 
-        if ($result) {
-            $this->afterUnrelate($entity, $relationName, $foreign, $options);
+        $this->afterUnrelate($entity, $relationName, $foreign, $options);
 
-            $afterMethodName = 'afterUnrelate' . ucfirst($relationName);
-            if (method_exists($this, $afterMethodName)) {
-                $this->$afterMethodName($entity, $foreign, $options);
-            }
+        $afterMethodName = 'afterUnrelate' . ucfirst($relationName);
+
+        if (method_exists($this, $afterMethodName)) {
+            $this->$afterMethodName($entity, $foreign, $options);
         }
 
         return $result;
@@ -596,7 +593,9 @@ class RDBRepository extends Repository
             throw new RuntimeException("Bad foreign value.");
         }
 
-        return $this->getMapper()->updateRelationColumns($entity, $relationName, $id, $columnData);
+        $this->getMapper()->updateRelationColumns($entity, $relationName, $id, $columnData);
+
+        return true;
     }
 
     /**
diff --git a/application/Espo/ORM/Repository/Repository.php b/application/Espo/ORM/Repository/Repository.php
index f745d6cd4a..e8016832c1 100644
--- a/application/Espo/ORM/Repository/Repository.php
+++ b/application/Espo/ORM/Repository/Repository.php
@@ -44,18 +44,13 @@ abstract class Repository
 
     protected $entityManager;
 
-    protected $seed;
-
     protected $entityType;
 
     public function __construct(string $entityType, EntityManager $entityManager, EntityFactory $entityFactory)
     {
         $this->entityType = $entityType;
-
         $this->entityFactory = $entityFactory;
         $this->entityManager = $entityManager;
-
-        $this->seed = $this->entityFactory->create($entityType);
     }
 
     protected function getEntityFactory(): EntityFactory