Question

If I have two models: Experience and Category which are a many to many association, that looks like this:

class Experience < ActiveRecord::Base
  has_and_belongs_to_many :categories
end


class Category < ActiveRecord::Base
  has_and_belongs_to_many :experiences
end

Why sometimes people add a:

attr_accessible :category_ids in the Experience model? I have found myself having to do that so in Rails admin gem I can add categories to a particular experience, but I fail to see why.

Answer 1

If you setup a HABTM relation between Experience and Category, you have, besides other things, available method Experience#category_ids. This method returns individual ids of categories the current experience has.

Experience.first.category_ids

Now this method has also Experience#category_ids= variant, so you can use it to assign ids of categories:

Experience.first.category_ids = [1, 2, 3]

Now when you have a form that let’s you select categories for an experience, you have only the ids of the selected categories. When you submit the form, those ids get passed to Experience#category_ids= via mass assigment and if you don’t have this category_ids in attr_accessible, you get an error.