Defining foreign key relationships for Rails’ models

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The Rails default behaviour is that the column used to hold the foreign key on a model is the name of the association with the suffix _id added. The :foreign_key option lets you set the name of the foreign key directly. The associations between your Post and Comment model classes should look like this:

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

—Note that you don’t need :class_name => "Post" in your Comment model. Rails already has that information. You should only be specifying :class_name and :foreign_key when you need to override the Rails’ conventions.

You’re correct that Rails maintains the foreign key relationships for you. You can enforce them in the database layer if you want by adding foreign key constraints.

  • I think you would benefit from reading A Guide to ActiveRecord Associations.

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