What's up with includes?

There's a strange interaction with ActiveRecord's includes and select and where. The SELECT portion of the query is being over-written when you put WHERE constraints on your query.

I made an example gist in accordance with Rails's guidelines for making Issues. Continue below if you want an interactive pry example so that you can play with it.

Set Up

Run bundle install then rake db:setup to get the DB right. After that, rake console will put you into a Pry session with useful classes loaded and will show you an example query.

What's Going On

The crux is that

User.select('random() AS ranking').order('ranking').includes(:comments).to_a

runs

SELECT random() AS ranking
FROM "users"
ORDER BY ranking

whereas

User.select('random() AS ranking').order('ranking').includes(:comments) \
  .where(comments: { id: 1 }).to_a

runs

SELECT "users"."id" AS t0_r0,
       "users"."name" AS t0_r1,
       "comments"."id" AS t1_r0,
       "comments"."body" AS t1_r1,
       "comments"."user_id" AS t1_r2
FROM "users"
LEFT OUTER JOIN "comments" ON "comments"."user_id" = "users"."id"
WHERE "comments"."id" = 1
ORDER BY ranking

Notice how ranking has been removed from the SELECT clause in the second query.