Question

In the Rails ActiveRecord Associations guide, I’m confused over why the tables for has_one and has_many are identical:

Example tables for has_many:

customers(id,name)
orders(id,customer_id,order_date)

Example tables for has_one:
these tables will, at the database level, also allow a supplier to have many accounts, but we just want one account per supplier

suppliers(id,name)
accounts(id,supplier_id,account_number) #Foreign Key to supplier here??

Shouldn’t the tables for has_one be like this instead:

suppliers(id,name,account_id) #Foreign Key to account here
accounts(id,account_number)

Now because the account_id is in the suppliers table, a supplier can never have more than one account.

Is the example in the Rails Guide incorrect?

Or, does Rails use the has_many kind of approach but restricts the many part from happening?

Answer 1

If I understand your question correctly, you believe there is a 1:1 relationship bi-directionally in a has_one/belongs_to relationship. That’s not exactly true. You could have:

Class Account
  belongs_to :supplier
  belongs_to :wholesaler
  belongs_to :shipper
  # ...
end

account = supplier.account       # Get supplier's account
wholesaler = Wholesaler.new
wholesaler.accounts << account   # Tell wholesaler this is one of their suppliers
wholesaler.save

I’m not saying your app actually behaves this way, but you can see how a table — no, let’s say a model — that “belongs to” another model is not precluded from belonging to any number of models. Right? So the relationship is really infinity:1.

I should add that has_one is really a degenerate case of has_many and just adds syntactic sugar of singularizing the association and a few other nits. Otherwise, it’s pretty much the same thing and it’s pretty much why they look alike.