带有子查询的Rails LEFT OUTER JOIN

Question

I have a simple calendar application.

Models:

class Day < ActiveRecord::Base
  self.primary_key = "sdate"
  has_many :items, foreign_key: :sdate
end

class Item < ActiveRecord::Base
  belongs_to :day, :foreign_key => :sdate
  belongs_to :calendar
end

class Calendar < ActiveRecord::Base
  has_many :items
end

Requirements: Fetch ALL rows of days within a certain date range. Fetch associated rows of items with certain Calendar-IDs.

With SQL I could use the following query:

SELECT * FROM days LEFT OUTER JOIN (
  SELECT * FROM items
  WHERE calendar_id IN (1, 4)
) AS items
ON days.sdate = items.sdate
WHERE days.sdate BETWEEN '2015-03-01' AND '2015-03-31';

I have tried several ways in Rails but I couldn't find a solution.

How can I do this in Rails 4? Eager loading would be great.

Answer 1

Something along these lines:

Day.joins('LEFT OUTER JOIN items ON days.sdate = items.sdate')
     .where('items.calendar_id IN (1, 4)')
     .where('days.sdate' => start_time..end_time)

http://guides.rubyonrails.org/active_record_querying.html

Answer 2

I was just trying to figure this out and I don't know of any way to do it with pure Rails. But this will do what you want:

Day.joins("LEFT OUTER JOIN (
  SELECT * FROM items
    WHERE calendar_id IN (1, 4)
  ) AS items
  ON days.sdate = items.sdate
  WHERE days.sdate BETWEEN '2015-03-01' AND '2015-03-31'");

I'm not using dates in the query in my code, so I'm not sure if you need the single quotes around the dates or not. But that is the syntax. I think Left joins are very handy, I wish Rails had an actual LEFT JOIN method.