Unknown constant when using habtm?


I have a Post AR and a Category AR. The Post has_and_belongs_to_many
:Categories. When I attempt to call destroy() on a post object, I am
getting the following error.

uninitalized Constant Post::Categories.

It seems as if it should be using Post::Category and not
Post::Categories. For my other relationships, there is a constant there
in singular form. (ex: a post has_many :Links and there is a constant
called Post::Link). There also exists a Post::Category constant…

Any ideas?