rails active record has_many foreign key after custom function

Question

Consider following models

class User < AR
   has_many :resources
end

class Resource < AR
   belongs_to :user
end 

I have a requirement where the foreign key is saved after applying some function on it. So the value of user_id in resources table does not match id in users table but it can be calculate again from id.

How can I define the association? Let's say that function is dummy_func().

Answer 1

Since belongs_to returns class instance instead of association, you can define methods in Resource class

class Resource < ApplicationRecord
  def user
    User.find(user_id)
  end

  def user=(user)
    user_id = user.id
  end
end

Similar for has_many result in user can be achieved by creating common relation in resources method

class User < ApplicationRecord
  def resources
    Resource.where(user_id: id)
  end
end

So, if you use this code, you can replace any ids in Resource model, and behavior will exactly same as in belongs_to (Maybe there is some difference in depths). And you can achieve very similar behavior in User model, by writing methods by yourself.