--- a/pylib/mozautomation/mozautomation/changetracker.py
+++ b/pylib/mozautomation/mozautomation/changetracker.py
@@ -95,16 +95,29 @@ class ChangeTracker(object):
                 return field[0]
 
             self._db.execute('INSERT INTO trees (name, url) VALUES (?, ?)',
                 [tree, url])
 
             return self._db.execute('SELECT id FROM trees WHERE name=? LIMIT 1',
                 [tree]).fetchone()[0]
 
+    def last_push_id(self, tree):
+        """Return the push ID of the last recorded push to a tree.
+
+        Returns an integer or None if no pushes are recorded.
+        """
+        tree, url = resolve_trees_to_uris([tree])[0]
+        tree_id = self.tree_id(tree, url)
+
+        last_push_id = self._db.execute('SELECT push_id FROM pushes WHERE '
+            'tree_id=? ORDER BY push_id DESC LIMIT 1', [tree_id]).fetchone()
+
+        return last_push_id[0] if last_push_id else None
+
     def load_pushlog(self, tree):
         tree, url = resolve_trees_to_uris([tree])[0]
         repo = MercurialRepository(url)
 
         tree_id = self.tree_id(tree, url)
 
         last_push_id = self._db.execute('SELECT push_id FROM pushes WHERE '
             'tree_id=? ORDER BY push_id DESC LIMIT 1', [tree_id]).fetchone()