Add two new accessors to ChangeSetItem

You shouldnt have to worry about exactly formatting the reference to
get a ChangeSetItem related to a DataObject, and doing it that way makes
you vulnerable to breakages like introduced in previous API change

model/versioning/ChangeSetItem.php

@@ -283,4 +283,30 @@ public function can($perm, $member = null, $context = array()) {
 		return (bool)Permission::checkMember($member, ChangeSet::config()->required_permission);
 	}
 
+	/**
+	 * Get the ChangeSetItems that reference a passed DataObject
+	 *
+	 * @param DataObject $object
+	 * @return DataList
+	 */
+	public static function get_for_object($object) {
+		return ChangeSetItem::get()->filter([
+			'ObjectID' => $object->ID,
+			'ObjectClass' => ClassInfo::baseDataClass($object)
+		]);
+	}
+
+	/**
+	 * Get the ChangeSetItems that reference a passed DataObject
+	 *
+	 * @param int $objectID The ID of the object
+	 * @param string $objectClass The class of the object (or any parent class)
+	 * @return DataList
+	 */
+	public static function get_for_object_by_id($objectID, $objectClass) {
+		return ChangeSetItem::get()->filter([
+			'ObjectID' => $objectID,
+			'ObjectClass' => ClassInfo::baseDataClass($objectClass)
+		]);
+	}
 }

tests/model/ChangeSetItemTest.php

@@ -73,4 +73,25 @@ function testChangeType() {
 			'Objects that have been deleted and then unpublished should return no change'
 		);
 	}
+
+	function testGetForObject() {
+		$object = new ChangeSetItemTest_Versioned(['Foo' => 1]);
+		$object->write();
+
+		$item = new ChangeSetItem([
+			'ObjectID' => $object->ID,
+			'ObjectClass' => ClassInfo::baseDataClass($object)
+		]);
+		$item->write();
+
+		$this->assertEquals(
+			ChangeSetItemTest_Versioned::get()->byID($object->ID)->toMap(),
+			ChangeSetItem::get_for_object($object)->first()->Object()->toMap()
+		);
+
+		$this->assertEquals(
+			ChangeSetItemTest_Versioned::get()->byID($object->ID)->toMap(),
+			ChangeSetItem::get_for_object_by_id($object->ID, $object->ClassName)->first()->Object()->toMap()
+		);
+	}
 }