Can two models belong_to each other?

Question

I have two models which are one-to-one to each other. A currently has one B.

Lately I encounter many cases where it is desirable if A keeps the id of B in order to simplify logic and boost performance. However I wonder if:

1. this is possible
2. would breach and convention
3. any thoughts really

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UPDATE

I was wrong, the left outer join would not be benefited by the extra foreign key.

The only place I can think of is to find all A's which does not have B, an inner join is required on each of my 100000+ records. But if there is an id then I can know straight away which A has a B.

Answer 1

I don't believe this is possible - you have to decide where to keep the foreign key. Would it make sense for you to use a joining through relationship and for both your existing models to be has_one through the join?

Answer 2

Another alternative is to put A id on B, ie denormalize. This would allow you to figure out which A's without a B, and A's with a B. This might be appropriate for reporting scenarios when B's don't move between A's often.