Recently I faced one scenario with NULL value with NOT IN, I have added the details here about what I learnt. suppose we have two table Category and Product Category id | name -------- 1 | Category 1 2 | Category 2 3 | Category 3 4 | Category 4 Product id | name | category_id ----------------------------- 1 | Product 1 | 1 2 | Product 2 | 2 3 | Product 3 | 2 4 | Product 4 | 3 5 | Product 5 | 4 6 | Product 6 | NULL Scenario :- *I just want to retrieve all the Products excluding Category 1 and Category 3 so I wrote following query :- Product.joins(:category)