ActiveRecord and might belongs_to

I have a pair of data types where every X may have many Y, and each Y has at most one X.

In the database, I'd visualized this as

 CREATE TABLE xs (
   id INTEGER NOT NULL PRIMARY KEY
 );
 CREATE TABLE ys (
   id IN开发者_运维知识库TEGER NOT NULL PRIMARY KEY,
   x_id INTEGER FOREIGN KEY REFERENCES xs (id) -- may be NULL
 );

Using ActiveRecord, it's easy for me to that every X has_many Y, but how do I express that every Y has at most one X? My impression that belongs_to would normally work, but I'm not sure how it'll like the situation when x_id is NULL.

---

You're right. Y should have a belongs_to :x.

If x_id is not present, y.x will return nil.

Having a belongs_to doesn't mean that if the value is not present, everything will blow up.