Recently I was working on a Rails project, and stumbled upon an interesting and hard-to-gooogle problem. I was looking for how to load records with the absense of an association, but the answers I was finding on StackOverflow we inadequate. I finally stumbled on the solution, and thought I should share for the next person.

I have two models, User and Post.

class User < ApplicationRecord
  has_one :post
end

class Post < ApplicationRecord
  belongs_to :user
end

The Post table holds the foreign key, and I want it to because in the future, the has_one could turn into a has_many.

select table_name, column_name
from information_schema.columns
where table_name = 'users' or table_name = 'posts';

 table_name | column_name
------------+-------------
 users      | id
 users      | email
 users      | password
 users      | created_at
 users      | updated_at
 posts      | id
 posts      | user_id
 posts      | created_at
 posts      | updated_at

Now the problem comes because I allow the user_id association on posts to be null, and at a certain point I want to associate a Post with the first User that doesn’t have one. Since the foreign key rightly lives on the Post we can’t just search on the null column:

user = User.where(post_id: nil).first

To solve this, we could just list all Users, excluding those which have been associated with:

user = User.where("id not in (select distinct(user_id) from posts)").first
# could also be expressed as
user = User.where("id not in (?)", Post.pluck(:user_id)).first

This can work on a mature table, but has a serious edge case: if I don’t have any rows in posts with a user_id, it returns no results! Let’s take a closer look at the query this generates to learn why:

SELECT  "users".* FROM "users"
WHERE (id not in (select distinct(user_id) from posts))
ORDER BY "users"."id" ASC LIMIT $1  [["LIMIT", 1]]

The subquery to gather user_ids to exclude, select distinct(user_id) from posts can return NULL given now rows, which in the context of the not in in the larger query makes no sense, and so no rows are returned.

Rather than living with a chicken and egg problem, we can use an outer join to get User rows matching our criteria:

select * from users
left join posts on posts.user_id = users.id
where posts.user_id is null;

In Rails 4, we would have to manually write this SQL:

User.join('left outer join posts on posts.user_id = users.id')
  .where(posts: {user_id: nil})
  .first

but Rails 5 makes this a little cleaner, and quite readble in my opinion:

User.left_joins(:posts)
  .where(posts: {user_id: nil})
  .first

Hopefully somebody will find this useful!

In the future, I hope we could get a further shorthand to drop the left_joins and where, maybe without, which could give us:

User.without(:post).first

Maybe I should pull request Rails, but dinner is getting cold.

Cam