Slow OR statement in postgresql

I currently have a postgresql query that is slow because of an OR statement. It's apparently not using an index because of it. Rewriting this query failed so far.

The query:

EXPLAIN ANALYZE SELECT a0_.id AS id0
FROM   advert a0_
       INNER JOIN advertcategory a1_
               ON a0_.advert_category_id = a1_.id
WHERE  a0_.advert_category_id IN ( 1136 )
        OR a1_.parent_id IN ( 1136 )
ORDER  BY a0_.created_date DESC
LIMIT  15;  

                                                                           QUERY PLAN                                                                                
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..27542.49 rows=15 width=12) (actual time=1.658..50.809 rows=15 loops=1)
   ->  Nested Loop  (cost=0.00..1691109.07 rows=921 width=12) (actual time=1.657..50.790 rows=15 loops=1)
         ->  Index Scan Backward using advert_created_date_idx on advert a0_  (cost=0.00..670300.17 rows=353804 width=16) (actual time=0.013..16.449 rows=12405 loops=1)
         ->  Index Scan using advertcategory_pkey on advertcategory a1_  (cost=0.00..2.88 rows=1 width=8) (actual time=0.002..0.002 rows=0 loops=12405)
               Index Cond: (id = a0_.advert_category_id)
               Filter: ((a0_.advert_category_id = 1136) OR (parent_id = 1136))
               Rows Removed by Filter: 1
 Total runtime: 50.860 ms

Cause of slowness: Filter: ((a0_.advert_category_id = 1136) OR (parent_id = 1136))

I've tried using INNER JOIN instead of the WHERE statement:

EXPLAIN ANALYZE  SELECT a0_.id AS id0
FROM   advert a0_
       INNER JOIN advertcategory a1_
               ON a0_.advert_category_id = a1_.id
                  AND ( a0_.advert_category_id IN ( 1136 )
                         OR a1_.parent_id IN ( 1136 ) )
ORDER  BY