SQL：使用OR链接排序范围（Ruby on Rails）

Question

I have 3 scopes on Professor that involves joining Professor::AvailabilityPeriod s, a model that represents professor availability to give classes at a given period of the week. This is achieved by storing the representation of the start and end time of each period as seconds from start of week, which I called starts_at_sfsow , and ends_at_sfsow .

Professor::AvailabilityPeriod has 2 scopes, overlapping(start_time, end_time) and contained_within(start_time, end_time) .

I have 3 scopes on Professor :

class Professor < ActiveRecord::Base
  scope :available_at, -> (opts) do
    start_time = opts.fetch(:start_time)
    end_time = opts.fetch(:end_time)

    includes(:availability_periods).
    joins(:availability_periods).merge(
      self::AvailabilityPeriod.overlaping(
        start_time: start_time,
        end_time: end_time,
      )
    )
  end

  scope :available_around, ->(opts) do
    start_time = opts.fetch(:start_time)
    end_time = opts.fetch(:end_time)

    available_at(
      start_time: start_time - 1.hour,
      end_time: start_time
    ).or available_at(
      start_time: end_time,
      end_time: end_time + 1.hour
    )
  end

  scope :partially_available_at, ->(opts) do
    start_time = opts.fetch(:start_time)
    end_time = opts.fetch(:end_time)

    includes(:availability_periods).
    joins(:availability_periods).merge(
      self::AvailabilityPeriod.contained_within(
        start_time: start_time,
        end_time: end_time,
      )
    )
  end
end

Then, I have a 4th scope that sums all the others, which is what I will use to search for professors in the controller (works fine):

  scope :available_at_or_around_or_partially, -> (opts) do
    distinct.
      available_at(opts).
      or(available_around(opts)).
      or(partially_available_at(opts))
  end

The problem: I need records returned by available_at to appear first, then available_around , and finally partially_available_at , in that order, and I also need to keep it pure SQL, so I can apply pagination later ; absolutely no in memory sorting.

I been thinking on using subqueries so I can do some kind of priority assignment to each scope like:

 distinct.
 where(id: available_at(opts).select("id, 1 AS priority")).   
 or(
   where(id: available_around(opts).select("id, 2 AS priority"))
 ). 
 or(
   where id: partially_available_at(opts).select("id, 3 AS priority")
 )
 order("priority, id")

... but this is not possible as postgres doesn't allow me to select more than 1 field per sub query, and that isn't event valid syntax.

BTW: if you are getting confused by the use of or method, that comes with where-or gem , a backport of Rails 5 functionality.

SQL :

Professor.available_at_or_around_or_partially start_time: Time.now, end_time: Time.now.+(2.hours)

SELECT     "professors".*
INNER JOIN "professor_availability_periods"
ON         "professor_availability_periods"."professor_id" = "professors"."id"
WHERE
(
  (
    /* available_at */
    ("professor_availability_periods"."starts_at_sfsow" <= 446340) /* start_time as seconds from start of week */
    AND ("professor_availability_periods"."ends_at_sfsow" >= 453540) /* end time as seconds from start of week*/

    /* available_around */
    OR (
      ("professor_availability_periods"."starts_at_sfsow" <= 442740)
      AND ("professor_availability_periods"."ends_at_sfsow" >= 446340)

      OR

      ("professor_availability_periods"."starts_at_sfsow" <= 453540)
      AND ("professor_availability_periods"."ends_at_sfsow" >= 457140)
    )
  )

  /* partially_available_at */
  OR ("professor_availability_periods"."starts_at_sfsow" >= 446340)
  AND ("professor_availability_periods"."ends_at_sfsow" <= 453540)
)

Answer 1

Been seeking the help of some colleagues and it seems that what I needed all this time was UNION ALL . This allows me to set <number> AS priority for each subquery and then ORDER BY priority . That's exactly what I need!

SQL :

SELECT DISTINCT "professors".* FROM (
  (SELECT "professors".* FROM
    (
      (SELECT *, 1 AS priority FROM "professors"
        INNER JOIN "professor_availability_periods"
        ON "professor_availability_periods"."professor_id" = "professors"."id"
        WHERE
          ("professor_availability_periods"."starts_at_sfsow" <= 455580)
          AND ("professor_availability_periods"."ends_at_sfsow" >= 462780)
      ) UNION ALL (
        SELECT *, 2 AS priority FROM "professors"
        INNER JOIN "professor_availability_periods"
        ON "professor_availability_periods"."professor_id" = "professors"."id"
        WHERE (
          ("professor_availability_periods"."starts_at_sfsow" <= 451980)
          AND ("professor_availability_periods"."ends_at_sfsow" >= 455580)
          OR
          ("professor_availability_periods"."starts_at_sfsow" <= 462780)
          AND ("professor_availability_periods"."ends_at_sfsow" >= 466380)
        )
      )
    ) professors
  ) UNION ALL (
    SELECT *, 3 AS priority FROM "professors"
    INNER JOIN "professor_availability_periods"
    ON "professor_availability_periods"."professor_id" = "professors"."id"
    WHERE
      ("professor_availability_periods"."starts_at_sfsow" >= 455580)
      AND ("professor_availability_periods"."ends_at_sfsow" <= 462780)
  )
) professors
ORDER BY priority

The function nesting looks a little awkward because it's generated by active_record_union , which in turn let's me perform this query through ActiveRecord as:

scope :search_by_availability, -> (opts) do
  available_at(opts).select("*", "1 AS priority").
  union_all(available_around(opts).select("*", "2 AS priority")).
  union_all(partially_available_at(opts).select("*", "3 AS priority")).
  order("priority").distinct(:id)
end