Rails: Create association if none is found to avoid nil errors

Question

I have an application where my users can have a set of preferences. Both are stored as ActiveRecord-models as follows:

class User < AR::Base
   has_one :p         


        

Answer 1

or simply

class User < ApplicationRecord
   has_one :preference_set

   def preference_set
     super || build_preference_set
   end
end

This works because ActiveRecord defines the association method in a mixin.

Answer 2

Well the best way to do this is to create the associated record when you create the primary one:

class User < ActiveRecord::Base
   has_one       :preference_set, :autosave => true
   before_create :build_preference_set
end

That will set it up so whenever a User is created, so is a PreferenceSet. If you need to initialise the the associated record with arguments, then call a different method in before_create which calls build_preference_set(:my_options => "here") and then returns true.

You can then just normalise all existing records by iterating over any that don't have a PreferenceSet and building one by calling #create_preference_set.

If you want to only create the PreferenceSet when it is absolutely needed, then you can do something like:

class User < ActiveRecord::Base
   has_one :preference_set

   def preference_set_with_initialize
     preference_set_without_initialize || build_preference_set
   end

   alias_method_chain :preference_set, :initialize
end