Question

The problem is that when a Restaurant does not have any MenuItems that match the condition, ActiveRecord says it can’t find the Restaurant. Here’s the relevant code:

class Restaurant < ActiveRecord::Base
  has_many :menu_items, dependent: :destroy
  has_many :meals, through: :menu_items

  def self.with_meals_of_the_week
    includes({menu_items: :meal}).where(:'menu_items.date' => Time.now.beginning_of_week..Time.now.end_of_week)
  end
end

And the sql code generated:

Restaurant Load (0.0ms)←[0m  ←[1mSELECT DISTINCT "restaurants".id FROM "restaurants"
LEFT OUTER JOIN "menu_items" ON "menu_items"."restaurant_id" = "restaurants"."id"
LEFT OUTER JOIN "meals" ON "meals"."id" = "menu_items"."meal_id" WHERE
"restaurants"."id" = ? AND ("menu_items"."date" BETWEEN '2012-10-14 23:00:00.000000'
AND '2012-10-21 22:59:59.999999') LIMIT 1←[0m  [["id", "1"]]

However, according to this part of the Rails Guides, this shouldn’t be happening:

Post.includes(:comments).where("comments.visible", true)

If, in the case of this includes query, there were no comments for any posts, all the posts would still be loaded.

Answer 1

The SQL generated is a correct translation of your query. But look at it,
just at the SQL level (i shortened it a bit):

SELECT *
FROM 
  "restaurants"
LEFT OUTER JOIN 
  "menu_items" ON "menu_items"."restaurant_id" = "restaurants"."id"
LEFT OUTER JOIN 
  "meals" ON "meals"."id" = "menu_items"."meal_id" 
WHERE 
  "restaurants"."id" = ? 
AND 
  ("menu_items"."date" BETWEEN '2012-10-14' AND '2012-10-21') 

the left outer joins do the work you expect them to do: restaurants
are combined with menu_items and meals; if there is no menu_item to
go with a restaurant, the restaurant is still kept in the result, with
all the missing pieces (menu_items.id, menu_items.date, …) filled in with NULL

now look aht the second part of the where: the BETWEEN operator demands,
that menu_items.date is not null! and this
is where you filter out all the restaurants without meals.

so we need to change the query in a way that makes having null-dates ok.

going back to ruby, you can write:

  def self.with_meals_of_the_week
    includes({menu_items: :meal})
       .where('menu_items.date is NULL or menu_items.date between ? and ?', 
            Time.now.beginning_of_week, 
            Time.now.end_of_week
        )
  end

The resulting SQL is now

.... WHERE (menu_items.date is NULL or menu_items.date between '2012-10-21' and '2012-10-28')

and the restaurants without meals stay in.