Revert some changes and allow the primary key to be an empty string

core-bundle/src/Resources/contao/library/Contao/Model.php

@@ -416,19 +416,6 @@ public function save()
 			throw new \RuntimeException('The model instance has been detached and cannot be saved');
 		}
 
-		$intPk = $this->{static::$strPk};
-
-		// Track primary key changes
-		if (isset($this->arrModified[static::$strPk]))
-		{
-			$intPk = $this->arrModified[static::$strPk];
-		}
-
-		if ($intPk !== null && empty($intPk))
-		{
-			throw new \RuntimeException('Model cannot be saved: Invalid primary key (' . static::$strPk . ')');
-		}
-
 		$objDatabase = \Database::getInstance();
 		$arrFields = $objDatabase->getFieldNames(static::$strTable);
 
@@ -456,6 +443,19 @@ public function save()
 				return $this;
 			}
 
+			$intPk = $this->{static::$strPk};
+
+			// Track primary key changes
+			if (isset($this->arrModified[static::$strPk]))
+			{
+				$intPk = $this->arrModified[static::$strPk];
+			}
+
+			if ($intPk === null)
+			{
+				throw new \RuntimeException('The primary key has not been set');
+			}
+
 			// Update the row
 			$objDatabase->prepare("UPDATE " . static::$strTable . " %s WHERE " . \Database::quoteIdentifier(static::$strPk) . "=?")
 						->set($arrSet)

core-bundle/src/Resources/contao/library/Contao/Model/Registry.php

@@ -168,11 +168,9 @@ public function register(Model $objModel)
 		$strPk = $objModel->getPk();
 		$varPk = $objModel->$strPk;
 
-		if ($varPk === null || $varPk === '')
+		if ($varPk === null)
 		{
-			throw new \RuntimeException(
-				"Cannot register a model with a primary key that is null or empty string ($strTable::$strPk)"
-			);
+			throw new \RuntimeException('The primary key has not been set');
 		}
 
 		// Another model object is pointing to the DB record already