Sometimes, in certain circumstances, it is reasonable to have a foreign key value of null.
ActiveRecord’s .has_many method has an argument to set the foreign key column on referencing rows to null when that record is deleted.
dependent: :nullify
Example:
class Post < ApplicationRecord
belongs_to :author
belongs_to :category
end
class Category < ApplicationRecord
has_many :posts, dependent: :nullify
end
In this example whenever a category is deleted, any posts referencing the categories table will have their foreign key set to null.
*References: http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_many-label-Options