diff --git a/model/DataObject.php b/model/DataObject.php
index 618ce6648..8381fbf3b 100644
--- a/model/DataObject.php
+++ b/model/DataObject.php
@@ -2077,7 +2077,7 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
 				$candidateManyManys = (array)Config::inst()->get($candidate, 'many_many', Config::UNINHERITED);
 
 				foreach($candidateManyManys as $relation => $relatedClass) {
-					if($relatedClass === $this->class) {
+					if (is_a($this, $relatedClass)) {
 						$relationName = $relation;
 					}
 				}
diff --git a/tests/model/ManyManyListExtensionTest.php b/tests/model/ManyManyListExtensionTest.php
new file mode 100644
index 000000000..4b37e7848
--- /dev/null
+++ b/tests/model/ManyManyListExtensionTest.php
@@ -0,0 +1,119 @@
+<?php
+
+/**
+ * @package framework
+ * @subpackage tests
+ */
+class ManyManyListExtensionTest extends SapphireTest {
+
+	protected static $fixture_file = 'ManyManyListExtensionTest.yml';
+
+	protected $extraDataObjects = array(
+		'ManyManyListTest_IndirectPrimary',
+		'ManyManyListTest_Secondary',
+		'ManyManyListTest_SecondarySub'
+	);
+
+	// Test that when one side of a many-many relationship is added by extension, both
+	// sides still see the extra fields.
+	public function testExtraFieldsViaExtension() {
+		// This extends ManyManyListTest_Secondary with the secondary extension that adds the relationship back
+		// to the primary. The instance from the fixture is ManyManyListTest_SecondarySub, deliberately a sub-class of
+		// the extended class.
+		Object::add_extension('ManyManyListTest_Secondary', 'ManyManyListTest_IndirectSecondaryExtension');
+
+		// Test from the primary (not extended) to the secondary (which is extended)
+		$primary = $this->objFromFixture('ManyManyListTest_IndirectPrimary', 'manymany_extra_primary');
+		$secondaries = $primary->Secondary();
+		$extraFields = $secondaries->getExtraFields();
+
+		$this->assertTrue(count($extraFields) > 0, 'has extra fields');
+		$this->assertTrue(isset($extraFields['DocumentSort']), 'has DocumentSort');
+
+		// Test from the secondary (which is extended) to the primary (not extended)
+		$secondary = $this->objFromFixture('ManyManyListTest_SecondarySub', 'manymany_extra_secondary');
+
+		$primaries = $secondary->Primary();
+		$extraFields = $primaries->getExtraFields();
+
+		$this->assertTrue(count($extraFields) > 0, 'has extra fields');
+		$this->assertTrue(isset($extraFields['DocumentSort']), 'has DocumentSort');
+	}
+}
+
+/**
+ * @package framework
+ * @subpackage tests
+ *
+ * A data object that implements the primary side of a many_many (where the extra fields are
+ * defined.) The many-many refers to ManyManyListTest_Secondary rather than ManyManyListTest_SecondarySub
+ * by design, because we're trying to test that a subclass instance picks up the extra fields of it's parent.
+ */
+class ManyManyListTest_IndirectPrimary extends DataObject implements TestOnly {
+
+	private static $db = array(
+		'Title' => 'Varchar(255)'
+	);
+
+	private static $many_many = array(
+		'Secondary' => 'ManyManyListTest_Secondary'
+	);
+
+	private static $many_many_extraFields = array(
+		'Secondary' => array(
+			'DocumentSort' => 'Int'
+		)
+	);
+}
+
+/**
+ * @package framework
+ * @subpackage tests
+ *
+ * A data object that implements the secondary side of a many_many when extended by
+ * ManyManyListTest_IndirectSecondaryExtension.
+ */
+class ManyManyListTest_Secondary extends DataObject implements TestOnly {
+
+	// Possibly not required, but want to simulate a real test failure case where
+	// database tables are present.
+	private static $db = array(
+		'Title' => 'Varchar(255)'
+	);
+
+}
+
+/**
+ * @package framework
+ * @subpackage tests
+ *
+ * A data object that is a subclass of the secondary side. The test will create an instance of this,
+ * and ensure that the extra fields are available on the instance even though the many many is
+ * defined at the parent level.
+ */
+class ManyManyListTest_SecondarySub extends ManyManyListTest_Secondary {
+
+	// private static $db = array(
+	// 	'Other' => 'Varchar(255)'
+	// );
+
+}
+
+/**
+ * @package framework
+ * @subpackage tests
+ *
+ * An extension that is applied to ManyManyListTest_Secondary that implements the other side of the many-many
+ * relationship.
+ */
+class ManyManyListTest_IndirectSecondaryExtension extends DataExtension implements TestOnly {
+
+	private static $db = array(
+		'Title' => 'Varchar(255)'
+	);
+
+	private static $belongs_many_many = array(
+		'Primary' => 'ManyManyListTest_IndirectPrimary'
+	);
+
+}
diff --git a/tests/model/ManyManyListExtensionTest.yml b/tests/model/ManyManyListExtensionTest.yml
new file mode 100644
index 000000000..a28c7be68
--- /dev/null
+++ b/tests/model/ManyManyListExtensionTest.yml
@@ -0,0 +1,6 @@
+ManyManyListTest_IndirectPrimary:
+  manymany_extra_primary:
+    Title: 'primary'
+ManyManyListTest_SecondarySub:
+  manymany_extra_secondary:
+    Title: 'secondary'