Rails BaseScopes and subclasses for tidying up excessive SQL scopes in models

base_scopes_sample.diff

+++ b/app/scopes/base_scopes.rb
+class BaseScopes < SimpleDelegator
+
+  def initialize
+    super self.class.relation
+  end
+
+  def self.method_missing(method_name, *args)
+    method = self.new.method(method_name)
+    method.to_proc
+  end
+
+  def self.relation
+    without_scopes_suffix = self.name.sub(/Scopes\z/, '')
+    without_scopes_suffix.constantize
+  end
+
+end

+++ b/app/scopes/event_scopes.rb
+class EventScopes < BaseScopes
+
+  def all_scope
+    joins("LEFT JOIN courses on events.course_id = courses.id").
+    joins("LEFT JOIN (
+      SELECT participants.event_id, participants.email
+      FROM (
+        SELECT event_id, MIN(id) AS first_instructor_id
+          FROM participants
+          WHERE type = 'Instructor'
+          GROUP BY event_id
+      ) AS event_id_to_first_instructor_id
+      JOIN participants
+        ON event_id_to_first_instructor_id.event_id = participants.event_id
+        AND event_id_to_first_instructor_id.first_instructor_id = participants.id
+    ) AS instructors
+    ON instructors.event_id = events.id").
+    select("events.*,
+      courses.name as course_name,
+      events.name as event_name,
+      instructors.email as first_instructor_email,
+      events.updated_at as event_updated_at
+    ")
+  end
+
+  def foo
+    where("start_at > ?", Time.zone.now)
+  end
+
+  def bar
+    where("end_at < ?", Time.zone.now)
+  end
+
+end

For background, read this article: http://craftingruby.com/posts/2015/06/29/query-objects-through-scopes.html

Solution derived from the above article plus an aim to reduce the boilerplate necessary in the Event and EventScopes class, such as avoiding needing to remember to use @relation in EventScopes and avoid adding an include or extend to Event. This solution minimizes boilerplate whilst still retaining the discoverability of preserving scope :foo, ... declarations in the model class.