diff --git a/application/Espo/Classes/Jobs/Cleanup.php b/application/Espo/Classes/Jobs/Cleanup.php
index 93f978b7ad..2dd556232f 100644
--- a/application/Espo/Classes/Jobs/Cleanup.php
+++ b/application/Espo/Classes/Jobs/Cleanup.php
@@ -612,7 +612,7 @@ class Cleanup implements JobDataLess
                     continue;
                 }
 
-                $midKey = $entity->getRelationParam($relation, 'midKeys')[0];
+                $midKey = $entity->getRelationParam($relation, RelationParam::MID_KEYS)[0];
 
                 if (!$midKey) {
                     continue;
diff --git a/application/Espo/Core/Upgrades/Migrations/V8_4/AfterUpgrade.php b/application/Espo/Core/Upgrades/Migrations/V8_4/AfterUpgrade.php
index dd8516fa31..792fc049a4 100644
--- a/application/Espo/Core/Upgrades/Migrations/V8_4/AfterUpgrade.php
+++ b/application/Espo/Core/Upgrades/Migrations/V8_4/AfterUpgrade.php
@@ -31,6 +31,7 @@ namespace Espo\Core\Upgrades\Migrations\V8_4;
 
 use Espo\Core\Upgrades\Migration\Script;
 use Espo\Core\Utils\Metadata;
+use Espo\ORM\Defs\Params\RelationParam;
 use Espo\ORM\Type\RelationType;
 
 class AfterUpgrade implements Script
@@ -58,7 +59,7 @@ class AfterUpgrade implements Script
             foreach ($item['links'] as $link => $linkDefs) {
                 $type = $linkDefs['type'] ?? null;
                 $foreignEntityType = $linkDefs['entity'] ?? null;
-                $midKeys = $linkDefs['midKeys'] ?? null;
+                $midKeys = $linkDefs[RelationParam::MID_KEYS] ?? null;
                 $isCustom = $linkDefs['isCustom'] ?? false;
 
                 if ($type !== RelationType::HAS_MANY) {
@@ -84,7 +85,7 @@ class AfterUpgrade implements Script
                 $this->metadata->set('entityDefs', $entityType, [
                     'links' => [
                         $link => [
-                            'midKeys' => array_reverse($midKeys),
+                            RelationParam::MID_KEYS => array_reverse($midKeys),
                             '_keysSwappedAfterUpgrade' => true,
                         ]
                     ]
diff --git a/application/Espo/Core/Utils/Database/Orm/Converter.php b/application/Espo/Core/Utils/Database/Orm/Converter.php
index 5da1a85a28..0693b36225 100644
--- a/application/Espo/Core/Utils/Database/Orm/Converter.php
+++ b/application/Espo/Core/Utils/Database/Orm/Converter.php
@@ -793,7 +793,7 @@ class Converter
 
                 $uniqueColumnList = [];
 
-                foreach (($relationData['midKeys'] ?? []) as $midKey) {
+                foreach (($relationData[RelationParam::MID_KEYS] ?? []) as $midKey) {
                     $indexName = $midKey;
 
                     $indexDefs = IndexDefs::fromRaw([IndexParam::COLUMNS => [$midKey]], $indexName);
diff --git a/application/Espo/Core/Utils/Database/Orm/Defs/RelationDefs.php b/application/Espo/Core/Utils/Database/Orm/Defs/RelationDefs.php
index 0a51f54b21..6fc31d4f0e 100644
--- a/application/Espo/Core/Utils/Database/Orm/Defs/RelationDefs.php
+++ b/application/Espo/Core/Utils/Database/Orm/Defs/RelationDefs.php
@@ -127,7 +127,7 @@ class RelationDefs
      */
     public function withKey(string $key): self
     {
-        return $this->withParam('key', $key);
+        return $this->withParam(RelationParam::KEY, $key);
     }
 
     /**
@@ -135,7 +135,7 @@ class RelationDefs
      */
     public function getKey(): ?string
     {
-        return $this->getParam('key');
+        return $this->getParam(RelationParam::KEY);
     }
 
     /**
@@ -143,7 +143,7 @@ class RelationDefs
      */
     public function withForeignKey(string $foreignKey): self
     {
-        return $this->withParam('foreignKey', $foreignKey);
+        return $this->withParam(RelationParam::FOREIGN_KEY, $foreignKey);
     }
 
     /**
@@ -151,7 +151,7 @@ class RelationDefs
      */
     public function getForeignKey(): ?string
     {
-        return $this->getParam('foreignKey');
+        return $this->getParam(RelationParam::FOREIGN_KEY);
     }
 
     /**
@@ -159,7 +159,7 @@ class RelationDefs
      */
     public function withMidKeys(string $midKey, string $foreignMidKey): self
     {
-        return $this->withParam('midKeys', [$midKey, $foreignMidKey]);
+        return $this->withParam(RelationParam::MID_KEYS, [$midKey, $foreignMidKey]);
     }
 
     /**
diff --git a/application/Espo/Modules/Crm/Tools/TargetList/OptOutService.php b/application/Espo/Modules/Crm/Tools/TargetList/OptOutService.php
index 65966036ac..939a4fde21 100644
--- a/application/Espo/Modules/Crm/Tools/TargetList/OptOutService.php
+++ b/application/Espo/Modules/Crm/Tools/TargetList/OptOutService.php
@@ -211,14 +211,14 @@ class OptOutService
         }
 
         $linkEntityType = ucfirst(
-            $seed->getRelationParam($link, 'relationName') ?? ''
+            $seed->getRelationParam($link, RelationParam::RELATION_NAME) ?? ''
         );
 
         if ($linkEntityType === '') {
             throw new RuntimeException();
         }
 
-        $key = $seed->getRelationParam($link, 'midKeys')[1] ?? null;
+        $key = $seed->getRelationParam($link, RelationParam::MID_KEYS)[1] ?? null;
 
         if (!$key) {
             throw new RuntimeException();
diff --git a/application/Espo/ORM/Defs/Params/RelationParam.php b/application/Espo/ORM/Defs/Params/RelationParam.php
index 37642ab03b..aa26d9670d 100644
--- a/application/Espo/ORM/Defs/Params/RelationParam.php
+++ b/application/Espo/ORM/Defs/Params/RelationParam.php
@@ -68,15 +68,14 @@ class RelationParam
      * Additional columns.
      */
     public const ADDITIONAL_COLUMNS = 'additionalColumns';
+
     /**
      * A key.
-     * @todo
      */
     public const KEY = 'key';
 
     /**
      * A foreign key.
-     * @todo
      */
     public const FOREIGN_KEY = 'foreignKey';
 
diff --git a/application/Espo/ORM/Defs/RelationDefs.php b/application/Espo/ORM/Defs/RelationDefs.php
index abc8dd17c5..f3a8f5c8b3 100644
--- a/application/Espo/ORM/Defs/RelationDefs.php
+++ b/application/Espo/ORM/Defs/RelationDefs.php
@@ -203,7 +203,7 @@ class RelationDefs
      */
     public function hasForeignKey(): bool
     {
-        return isset($this->data['foreignKey']);
+        return isset($this->data[RelationParam::FOREIGN_KEY]);
     }
 
     /**
@@ -217,7 +217,7 @@ class RelationDefs
             throw new RuntimeException("No 'foreignKey' parameter defined in the relation '{$this->name}'.");
         }
 
-        return $this->data['foreignKey'];
+        return $this->data[RelationParam::FOREIGN_KEY];
     }
 
     /**
@@ -225,7 +225,7 @@ class RelationDefs
      */
     public function hasKey(): bool
     {
-        return isset($this->data['key']);
+        return isset($this->data[RelationParam::KEY]);
     }
 
     /**
@@ -238,7 +238,7 @@ class RelationDefs
             throw new RuntimeException("No 'key' parameter defined in the relation '{$this->name}'.");
         }
 
-        return $this->data['key'];
+        return $this->data[RelationParam::KEY];
     }
 
     /**
@@ -246,7 +246,7 @@ class RelationDefs
      */
     public function hasMidKey(): bool
     {
-        return !is_null($this->data['midKeys'][0] ?? null);
+        return !is_null($this->data[RelationParam::MID_KEYS][0] ?? null);
     }
 
     /**
@@ -260,7 +260,7 @@ class RelationDefs
             throw new RuntimeException("No 'midKey' parameter defined in the relation '{$this->name}'.");
         }
 
-        return $this->data['midKeys'][0];
+        return $this->data[RelationParam::MID_KEYS][0];
     }
 
     /**
@@ -270,7 +270,7 @@ class RelationDefs
      */
     public function hasForeignMidKey(): bool
     {
-        return !is_null($this->data['midKeys'][1] ?? null);
+        return !is_null($this->data[RelationParam::MID_KEYS][1] ?? null);
     }
 
     /**
@@ -284,7 +284,7 @@ class RelationDefs
             throw new RuntimeException("No 'foreignMidKey' parameter defined in the relation '{$this->name}'.");
         }
 
-        return $this->data['midKeys'][1];
+        return $this->data[RelationParam::MID_KEYS][1];
     }
 
     /**
diff --git a/application/Espo/Tools/LinkManager/LinkManager.php b/application/Espo/Tools/LinkManager/LinkManager.php
index ca3dce65d9..dd57200bcf 100644
--- a/application/Espo/Tools/LinkManager/LinkManager.php
+++ b/application/Espo/Tools/LinkManager/LinkManager.php
@@ -38,6 +38,8 @@ use Espo\Core\Utils\Language;
 use Espo\Core\Utils\Metadata;
 use Espo\Core\Utils\Route;
 use Espo\Core\Utils\Util;
+use Espo\ORM\Defs\Params\EntityParam;
+use Espo\ORM\Defs\Params\FieldParam;
 use Espo\ORM\Defs\Params\RelationParam;
 use Espo\ORM\Entity;
 use Espo\ORM\Type\RelationType;
@@ -510,23 +512,23 @@ class LinkManager
                 }
 
                 if ($entityForeign == $entity) {
-                    $dataLeft['links'][$link]['midKeys'] = ['leftId', 'rightId'];
-                    $dataRight['links'][$linkForeign]['midKeys'] = ['rightId', 'leftId'];
+                    $dataLeft['links'][$link][RelationParam::MID_KEYS] = ['leftId', 'rightId'];
+                    $dataRight['links'][$linkForeign][RelationParam::MID_KEYS] = ['rightId', 'leftId'];
                 }
 
                 break;
 
             case self::CHILDREN_TO_PARENT:
                 $dataLeft = [
-                    'fields' => [
+                    EntityParam::FIELDS => [
                         $link => [
-                            'type' => FieldType::LINK_PARENT,
+                            FieldParam::TYPE => FieldType::LINK_PARENT,
                             'entityList' => $params['parentEntityTypeList'] ?? null,
                         ],
                     ],
                     'links' => [
                         $link => [
-                            'type' => Entity::BELONGS_TO_PARENT,
+                            RelationParam::TYPE => Entity::BELONGS_TO_PARENT,
                             'foreign' => $linkForeign,
                             'isCustom' => true,
                         ],