diff --git a/src/Symfony/Bridge/Doctrine/PropertyInfo/DoctrineExtractor.php b/src/Symfony/Bridge/Doctrine/PropertyInfo/DoctrineExtractor.php
index ecd25088be0f..6e8c29e16b0d 100644
--- a/src/Symfony/Bridge/Doctrine/PropertyInfo/DoctrineExtractor.php
+++ b/src/Symfony/Bridge/Doctrine/PropertyInfo/DoctrineExtractor.php
@@ -71,7 +71,9 @@ public function getTypes($class, $property, array $context = array())
 
             if ($metadata->isSingleValuedAssociation($property)) {
                 if ($metadata instanceof ClassMetadataInfo) {
-                    $nullable = isset($metadata->discriminatorColumn['nullable']) ? $metadata->discriminatorColumn['nullable'] : false;
+                    $associationMapping = $metadata->getAssociationMapping($property);
+
+                    $nullable = $this->isAssociationNullable($associationMapping);
                 } else {
                     $nullable = false;
                 }
@@ -79,12 +81,25 @@ public function getTypes($class, $property, array $context = array())
                 return array(new Type(Type::BUILTIN_TYPE_OBJECT, $nullable, $class));
             }
 
+            $collectionKeyType = Type::BUILTIN_TYPE_INT;
+
+            if ($metadata instanceof ClassMetadataInfo) {
+                $associationMapping = $metadata->getAssociationMapping($property);
+
+                if (isset($associationMapping['indexBy'])) {
+                    $indexProperty = $associationMapping['indexBy'];
+                    $typeOfField = $metadata->getTypeOfField($indexProperty);
+
+                    $collectionKeyType = $this->getPhpType($typeOfField);
+                }
+            }
+
             return array(new Type(
                 Type::BUILTIN_TYPE_OBJECT,
                 false,
                 'Doctrine\Common\Collections\Collection',
                 true,
-                new Type(Type::BUILTIN_TYPE_INT),
+                new Type($collectionKeyType),
                 new Type(Type::BUILTIN_TYPE_OBJECT, false, $class)
             ));
         }
@@ -118,6 +133,35 @@ public function getTypes($class, $property, array $context = array())
         }
     }
 
+    /**
+     * Determines whether an association is nullable.
+     *
+     * @param array $associationMapping
+     *
+     * @return bool
+     *
+     * @see https://github.com/doctrine/doctrine2/blob/v2.5.4/lib/Doctrine/ORM/Tools/EntityGenerator.php#L1221-L1246
+     */
+    private function isAssociationNullable(array $associationMapping)
+    {
+        if (isset($associationMapping['id']) && $associationMapping['id']) {
+            return false;
+        }
+
+        if (!isset($associationMapping['joinColumns'])) {
+            return true;
+        }
+
+        $joinColumns = $associationMapping['joinColumns'];
+        foreach ($joinColumns as $joinColumn) {
+            if (isset($joinColumn['nullable']) && !$joinColumn['nullable']) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
     /**
      * Gets the corresponding built-in PHP type.
      *
diff --git a/src/Symfony/Bridge/Doctrine/Tests/PropertyInfo/DoctrineExtractorTest.php b/src/Symfony/Bridge/Doctrine/Tests/PropertyInfo/DoctrineExtractorTest.php
index ca098b6fc8e7..1bc57b1140fa 100644
--- a/src/Symfony/Bridge/Doctrine/Tests/PropertyInfo/DoctrineExtractorTest.php
+++ b/src/Symfony/Bridge/Doctrine/Tests/PropertyInfo/DoctrineExtractorTest.php
@@ -54,6 +54,7 @@ public function testGetProperties()
                 'customFoo',
                 'foo',
                 'bar',
+                'indexedBar',
             ),
             $this->extractor->getProperties('Symfony\Bridge\Doctrine\Tests\PropertyInfo\Fixtures\DoctrineDummy')
         );
@@ -75,7 +76,7 @@ public function typesProvider()
             array('bool', array(new Type(Type::BUILTIN_TYPE_BOOL))),
             array('binary', array(new Type(Type::BUILTIN_TYPE_RESOURCE))),
             array('json', array(new Type(Type::BUILTIN_TYPE_ARRAY, false, null, true))),
-            array('foo', array(new Type(Type::BUILTIN_TYPE_OBJECT, false, 'Symfony\Bridge\Doctrine\Tests\PropertyInfo\Fixtures\DoctrineRelation'))),
+            array('foo', array(new Type(Type::BUILTIN_TYPE_OBJECT, true, 'Symfony\Bridge\Doctrine\Tests\PropertyInfo\Fixtures\DoctrineRelation'))),
             array('bar', array(new Type(
                 Type::BUILTIN_TYPE_OBJECT,
                 false,
@@ -84,6 +85,14 @@ public function typesProvider()
                 new Type(Type::BUILTIN_TYPE_INT),
                 new Type(Type::BUILTIN_TYPE_OBJECT, false, 'Symfony\Bridge\Doctrine\Tests\PropertyInfo\Fixtures\DoctrineRelation')
             ))),
+            array('indexedBar', array(new Type(
+                Type::BUILTIN_TYPE_OBJECT,
+                false,
+                'Doctrine\Common\Collections\Collection',
+                true,
+                new Type(Type::BUILTIN_TYPE_STRING),
+                new Type(Type::BUILTIN_TYPE_OBJECT, false, 'Symfony\Bridge\Doctrine\Tests\PropertyInfo\Fixtures\DoctrineRelation')
+            ))),
             array('simpleArray', array(new Type(Type::BUILTIN_TYPE_ARRAY, false, null, true, new Type(Type::BUILTIN_TYPE_INT), new Type(Type::BUILTIN_TYPE_STRING)))),
             array('customFoo', null),
             array('notMapped', null),
diff --git a/src/Symfony/Bridge/Doctrine/Tests/PropertyInfo/Fixtures/DoctrineDummy.php b/src/Symfony/Bridge/Doctrine/Tests/PropertyInfo/Fixtures/DoctrineDummy.php
index 0b6b7bb54403..317d14257b20 100644
--- a/src/Symfony/Bridge/Doctrine/Tests/PropertyInfo/Fixtures/DoctrineDummy.php
+++ b/src/Symfony/Bridge/Doctrine/Tests/PropertyInfo/Fixtures/DoctrineDummy.php
@@ -40,6 +40,11 @@ class DoctrineDummy
      */
     public $bar;
 
+    /**
+     * @ManyToMany(targetEntity="DoctrineRelation", indexBy="guid")
+     */
+    protected $indexedBar;
+
     /**
      * @Column(type="guid")
      */
diff --git a/src/Symfony/Bridge/Doctrine/Tests/PropertyInfo/Fixtures/DoctrineRelation.php b/src/Symfony/Bridge/Doctrine/Tests/PropertyInfo/Fixtures/DoctrineRelation.php
index bfb27e9338d9..23f64d829e03 100644
--- a/src/Symfony/Bridge/Doctrine/Tests/PropertyInfo/Fixtures/DoctrineRelation.php
+++ b/src/Symfony/Bridge/Doctrine/Tests/PropertyInfo/Fixtures/DoctrineRelation.php
@@ -12,6 +12,7 @@
 namespace Symfony\Bridge\Doctrine\Tests\PropertyInfo\Fixtures;
 
 use Doctrine\ORM\Mapping\Column;
+use Doctrine\ORM\Mapping\Entity;
 use Doctrine\ORM\Mapping\Id;
 
 /**
@@ -26,4 +27,9 @@ class DoctrineRelation
      * @Column(type="smallint")
      */
     public $id;
+
+    /**
+     * @Column(type="guid")
+     */
+    protected $guid;
 }