laravel/framework — pull request #2969 by anlutro (2013-12-16 17:31:38+00:00)
Title: Disassociate on BelongsTo

Currently you have to manually set the foreign key attribute to null, which is annoying if you later down the road decide to change the foreign key. A disassociate method is a simple, non-intrusive way to solve this small issue.