Daniel Strunk

"references" vs "belongs_to" in ActiveRecord migrations

I always wondered about the difference between :belongs_to vs :references when running ActiveRecord migrations, always chalking it up to a quirk in the framework. They do the same thing, right? Well yes, they do exactly the same thing, as a matter of fact; take a look at the Rails source:

def references(*args)
	  options = args.extract_options!
	  polymorphic = options.delete(:polymorphic)
	  args.each do |col|
	    column("#{col}_id", :integer, options)
	    column("#{col}_type", :string, polymorphic.is_a?(Hash) ? polymorphic : options) unless polymorphic.nil?
	alias :belongs_to :references

But while they do the same thing, they might make your code a lot more readable to others … or at least help you to understand why you’re doing the association in the first place. Consider these two examples:

Answers belong_to Questions

Say we have two models, Question and Answer:

class Question < AR::Base
	has_many :answers

class Answer < AR::Base
	belongs_to :question

In this case, when creating the answer AR migration, it would make sense for me to use t.belongs_to :question. From a vernacular standpoint, it’s obvious that an answer instance belongs directly to a question.

Questions reference Topics

But in a relationship that isn’t strictly “parent and child” per-say, it might make more sense in your brain to use references. Again, this is purely from a vernacular standpoint; there are no differences once the table is created. My example uses Questions and Topics:

class Question < AR::Base
	belongs_to :topic

class Topic < AR::Base
	has_many :questions

There’s no difference in the model, but if I were creating the table, I’d use t.references :topic. This would help make my migration more readable, and I’d know that Topic is probably a more adjacent table that Question just happens to need a reference to, instead of being a more “connected”, “parent-child” relationship that might directly affect the child models. I tend to use belongs_to most when I’m using models that accept nested attributes for another model. reference is more for tenuous relationships that just need to “talk” to another table, or are related via a has_many through, personally. Notice all of the qualifiers in this post! The reason for all of the “personally” or “I tend to” is because doesn’t really matter which you use: belongs_to or references. It might help make your migrations more readable, however. It certainly helps me! Have anything to add? Want to correct me on something? Let me know below!