title: Laravel 5 Deleting a one-to-many relationship
body: <p>I have two tables:
   - Products
   - Themes</p>
  
  <p>Products belong to a Theme - Themes has many Products.</p>
  
  <p>I want to delete a theme and remove it's association with any Products. In a perfect world deleting a theme would reset the related Products' theme_id to NULL. In my Products table I have tried ->onDelete('cascade') but this deletes both the Theme and corresponding Products. If I do not implement ->onDelete('cascade') I get this error:</p>
  
  <p>QLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (<code>acme</code>.<code>products</code>, CONSTRAINT <code>products_theme_id_foreign</code> FOREIGN KEY (<code>theme_id</code>) REFERENCES <code>themes</code> (<code>id</code>) ON DELETE CASCADE) (SQL: update <code>products</code> set <code>theme_id</code> = , <code>updated_at</code> = 2015-12-28 20:20:05 where <code>id</code> = 1)</p>
  
  <p>Any suggestions would be super helpful. Thank You!</p>