linkOnlyNotLinked param

application/Espo/Core/Record/Access/LinkCheck.php

@@ -320,6 +320,17 @@ class LinkCheck
      * @throws Forbidden
      */
     public function processLinkForeign(Entity $entity, string $link, Entity $foreignEntity): void
+    {
+        $this->processLinkForeignInternal($entity, $link, $foreignEntity);
+        $this->processLinkAlreadyLinkedCheck($entity, $link, $foreignEntity);
+    }
+
+    /**
+     * Check link access for a specific foreign entity.
+     *
+     * @throws Forbidden
+     */
+    private function processLinkForeignInternal(Entity $entity, string $link, Entity $foreignEntity): void
     {
         $toSkip = $this->linkForeignAccessCheckMany($entity->getEntityType(), $link, $foreignEntity);
 
@@ -337,7 +348,7 @@ class LinkCheck
      */
     public function processUnlinkForeign(Entity $entity, string $link, Entity $foreignEntity): void
     {
-        $this->processLinkForeign($entity, $link, $foreignEntity);
+        $this->processLinkForeignInternal($entity, $link, $foreignEntity);
         $this->processUnlinkForeignRequired($entity, $link, $foreignEntity);
     }
 
@@ -697,4 +708,41 @@ class LinkCheck
 
         return $names;
     }
+
+    /**
+     * @throws Forbidden
+     */
+    private function processLinkAlreadyLinkedCheck(Entity $entity, string $link, Entity $foreignEntity): void
+    {
+        if (!$this->getParam($entity->getEntityType(), $link, 'linkOnlyNotLinked')) {
+            return;
+        }
+
+        $entityType = $entity->getEntityType();
+
+        $foreign = $this->ormDefs
+            ->getEntity($entityType)
+            ->tryGetRelation($link)
+            ?->tryGetForeignRelationName();
+
+        if (!$foreign) {
+            return;
+        }
+
+        $one = $this->entityManager
+            ->getRDBRepository($foreignEntity->getEntityType())
+            ->getRelation($foreignEntity, $foreign)
+            ->findOne();
+
+        if (!$one) {
+            return;
+        }
+
+        throw ForbiddenSilent::createWithBody(
+            "Cannot link as the record is already linked ($entityType:$link).",
+            ErrorBody::create()
+                ->withMessageTranslation('cannotLinkAlreadyLinked')
+                ->encode()
+        );
+    }
 }

application/Espo/Modules/Crm/Resources/metadata/recordDefs/Account.json

@@ -6,6 +6,9 @@
         },
         "targetLists": {
             "mandatoryAttributeList": ["isOptedOut"]
+        },
+        "opportunities": {
+            "linkOnlyNotLinked": true
         }
     }
 }

application/Espo/ORM/Defs/RelationDefs.php

@@ -169,6 +169,20 @@ class RelationDefs
         return isset($this->data['foreign']);
     }
 
+    /**
+     * Try to get a foreign relation name.
+     *
+     * @since 8.3.0
+     */
+    public function tryGetForeignRelationName(): ?string
+    {
+        if (!$this->hasForeignRelationName()) {
+            return null;
+        }
+
+        return $this->getForeignRelationName();
+    }
+
     /**
      * Get a foreign relation name.
      *

application/Espo/Resources/i18n/en_US/Global.json

@@ -397,6 +397,7 @@
         "cannotRelateNonExisting": "Can't relate with non-existing {foreignEntityType} record.",
         "cannotRelateForbidden": "Can't relate with forbidden {foreignEntityType} record. `{action}` access required.",
         "cannotRelateForbiddenLink": "No access to link '{link}'.",
+        "cannotLinkAlreadyLinked": "Cannot link an already linked record.",
         "error404": "The url you requested can't be handled.",
         "error403": "You don't have an access to this area.",
         "emptyMassUpdate": "No fields available for Mass Update.",

schema/metadata/recordDefs.json

@@ -158,6 +158,10 @@
                     "massLink": {
                         "type": "boolean",
                         "default": "Allow mass link. As of v8.2."
+                    },
+                    "linkOnlyNotLinked": {
+                        "type": "boolean",
+                        "description": "Allow linking only if a record is not linked with any. As of v8.3."
                     }
                 }
             }