Fix notice in ClassMetadata when there is no ID Column defined

When you forget about defining the ID/PK Column, then this ugly Notice appear.
Now it will throw nice Exception.

lib/Doctrine/ORM/Mapping/ClassMetadataInfo.php

@@ -1796,13 +1796,17 @@ class ClassMetadataInfo implements ClassMetadata
      *
      * @return string
      *
-     * @throws MappingException If the class has a composite primary key.
+     * @throws MappingException If the class doesn't have an identifier or it has a composite primary key.
      */
     public function getSingleIdentifierFieldName()
     {
         if ($this->isIdentifierComposite) {
             throw MappingException::singleIdNotAllowedOnCompositePrimaryKey($this->name);
         }
+        
+        if ( ! isset($this->identifier[0])) {
+            throw MappingException::noIdDefined($this->name);
+        }
 
         return $this->identifier[0];
     }
@@ -1813,7 +1817,7 @@ class ClassMetadataInfo implements ClassMetadata
      *
      * @return string
      *
-     * @throws MappingException If the class has a composite primary key.
+     * @throws MappingException If the class doesn't have an identifier or it has a composite primary key.
      */
     public function getSingleIdentifierColumnName()
     {

lib/Doctrine/ORM/Mapping/MappingException.php

@@ -424,6 +424,16 @@ class MappingException extends \Doctrine\ORM\ORMException
         return new self('Single id is not allowed on composite primary key in entity '.$entity);
     }
 
+    /**
+     * @param string $entity
+     *
+     * @return MappingException
+     */
+    public static function noIdDefined($entity)
+    {
+        return new self('No ID defined for entity ' . $entity);
+    }
+
     /**
      * @param string $entity
      * @param string $fieldName

tests/Doctrine/Tests/Models/DDC6412/DDC6412File.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace Doctrine\Tests\Models\DDC6412;
+
+/**
+ * @Entity
+ */
+class DDC6412File
+{
+    /**
+     * @Column(type="integer")
+     * @GeneratedValue
+     */
+    public $id;
+    
+    /**
+     * @Column(length=50, name="file_name")
+     */
+    public $name;
+}
+

tests/Doctrine/Tests/ORM/Mapping/ClassMetadataTest.php

@@ -14,6 +14,7 @@ use Doctrine\Tests\Models\Company\CompanyContract;
 use Doctrine\Tests\Models\CustomType\CustomTypeParent;
 use Doctrine\Tests\Models\DDC117\DDC117Article;
 use Doctrine\Tests\Models\DDC117\DDC117ArticleDetails;
+use Doctrine\Tests\Models\DDC6412\DDC6412File;
 use Doctrine\Tests\Models\DDC964\DDC964Admin;
 use Doctrine\Tests\Models\DDC964\DDC964Guest;
 use Doctrine\Tests\Models\Routing\RoutingLeg;
@@ -209,6 +210,15 @@ class ClassMetadataTest extends OrmTestCase
         $cm->getSingleIdentifierFieldName();
     }
 
+    public function testGetSingleIdentifierFieldName_NoIdEntity_ThrowsException()
+    {
+        $cm = new ClassMetadata(DDC6412File::class);
+        $cm->initializeReflection(new RuntimeReflectionService());
+
+        $this->expectException(\Doctrine\ORM\Mapping\MappingException::class);
+        $cm->getSingleIdentifierFieldName();
+    }
+
     public function testDuplicateAssociationMappingException()
     {
         $cm = new ClassMetadata(CMS\CmsUser::class);