How do I associate my objects so that a many-to-many relation can be used as one-to-many in Rails?

Please excuse the confusing phrasing in the title. In my RoR project let's say I have it set up like this

class Product < ActiveRecord::Base
  has_and_belongs_to_many :categories
end

and

class Category < ActiveRecord::Base
  has_and_belongs_to_many :products
end

I then have a categories_products table that connects them. This works fine but my problem is that a product will only ever have one category at a time and I'd of course like to do 开发者_高级运维product.category instead of having to deal with an array. How can I accomplish that?

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A one-to-many representation is demonstrated in the rails guides like this:

class Category < ActiveRecord::Base
  has_many :products
end

class Product < ActiveRecord::Base
  belongs_to :category
end