Sample for self joins

gistfile1.rb

class Event < ActiveRecord::Base
  has_many :groups

  # reference: http://guides.rubyonrails.org/association_basics.html#self-joins
  has_many :next_events, class_name: "Event", foreign_key: "previous_event_id"
  belongs_to :previous_event, class_name: "Event"
end

# schema.rb
  create_table "events", force: :cascade do |t|
    t.string   "name"
    t.datetime "created_at",        null: false
    t.datetime "updated_at",        null: false
    t.integer  "previous_event_id"
    t.integer  "next_event_id"
  end

# rails console
irb(main):029:0> Event.joins(:previous_event).where(previous_event: {name: 'prev'})
  Event Load (0.3ms)  SELECT "events".* FROM "events" INNER JOIN "events" "previous_events_events" ON "previous_events_events"."id" = "events"."previous_event_id" WHERE "previous_events_events"."name" = ?  [["name", "prev"]]
=> #<ActiveRecord::Relation [#<Event id: 1, name: "current", created_at: "2015-02-11 12:38:38", updated_at: "2015-02-15 21:09:01", previous_event_id: 2, next_event_id: nil>]>
irb(main):030:0> Event.joins(:previous_event).where(previous_event: {name: 'prevx'})
  Event Load (0.1ms)  SELECT "events".* FROM "events" INNER JOIN "events" "previous_events_events" ON "previous_events_events"."id" = "events"."previous_event_id" WHERE "previous_events_events"."name" = ?  [["name", "prevx"]]
=> #<ActiveRecord::Relation []>
irb(main):031:0> Event.includes(:previous_event).references(:previous_event).where(previous_event: {name: 'prevx'})
  SQL (0.3ms)  SELECT "events"."id" AS t0_r0, "events"."name" AS t0_r1, "events"."created_at" AS t0_r2, "events"."updated_at" AS t0_r3, "events"."previous_event_id" AS t0_r4, "events"."next_event_id" AS t0_r5, "previous_events_events"."id" AS t1_r0, "previous_events_events"."name" AS t1_r1, "previous_events_events"."created_at" AS t1_r2, "previous_events_events"."updated_at" AS t1_r3, "previous_events_events"."previous_event_id" AS t1_r4, "previous_events_events"."next_event_id" AS t1_r5 FROM "events" LEFT OUTER JOIN "events" "previous_events_events" ON "previous_events_events"."id" = "events"."previous_event_id" WHERE "previous_events_events"."name" = ?  [["name", "prevx"]]
=> #<ActiveRecord::Relation []>
irb(main):032:0> Event.includes(:previous_event).references(:previous_event).where(previous_event: {name: 'prev'})
  SQL (0.1ms)  SELECT "events"."id" AS t0_r0, "events"."name" AS t0_r1, "events"."created_at" AS t0_r2, "events"."updated_at" AS t0_r3, "events"."previous_event_id" AS t0_r4, "events"."next_event_id" AS t0_r5, "previous_events_events"."id" AS t1_r0, "previous_events_events"."name" AS t1_r1, "previous_events_events"."created_at" AS t1_r2, "previous_events_events"."updated_at" AS t1_r3, "previous_events_events"."previous_event_id" AS t1_r4, "previous_events_events"."next_event_id" AS t1_r5 FROM "events" LEFT OUTER JOIN "events" "previous_events_events" ON "previous_events_events"."id" = "events"."previous_event_id" WHERE "previous_events_events"."name" = ?  [["name", "prev"]]
=> #<ActiveRecord::Relation [#<Event id: 1, name: "current", created_at: "2015-02-11 12:38:38", updated_at: "2015-02-15 21:09:01", previous_event_id: 2, next_event_id: nil>]>

# with squeel
irb(main):034:0> Event.joins{previous_event.outer}.where{previous_event.name == 'prev'}
  Event Load (2.0ms)  SELECT "events".* FROM "events" LEFT OUTER JOIN "events" "previous_events_events" ON "previous_events_events"."id" = "events"."previous_event_id" WHERE "previous_events_events"."name" = 'prev'
=> #<ActiveRecord::Relation [#<Event id: 1, name: "current", created_at: "2015-02-11 12:38:38", updated_at: "2015-02-15 21:09:01", previous_event_id: 2, next_event_id: nil>]>
irb(main):035:0> Event.joins{previous_event}.where{previous_event.name == 'prev'}
  Event Load (0.3ms)  SELECT "events".* FROM "events" INNER JOIN "events" "previous_events_events" ON "previous_events_events"."id" = "events"."previous_event_id" WHERE "previous_events_events"."name" = 'prev'
=> #<ActiveRecord::Relation [#<Event id: 1, name: "current", created_at: "2015-02-11 12:38:38", updated_at: "2015-02-15 21:09:01", previous_event_id: 2, next_event_id: nil>]>

# ==============

class Event < ActiveRecord::Base
  has_many :groups

  has_many :next_events, class_name: "Event", foreign_key: "previous_event_id"
  belongs_to :previous_event, class_name: "Event"

  has_many :previous_events, class_name: "Event", foreign_key: "next_event_id"
  belongs_to :next_event, class_name: "Event"
end

  create_table "events", force: :cascade do |t|
    t.string   "name"
    t.datetime "created_at",        null: false
    t.datetime "updated_at",        null: false
    t.integer  "previous_event_id"
    t.integer  "next_event_id"
  end

>> Event.joins(:previous_event, :next_event) 
#<ActiveRecord::Relation [#<Event id: 1, name: "current", created_at: "2015-02-11 12:38:38", updated_at: "2015-02-17 03:35:28", previous_event_id: 2, next_event_id: 6>]>

>> Event.joins(:previous_event, :next_event).to_sql 
"SELECT \"events\".* FROM \"events\" INNER JOIN \"events\" \"previous_events_events\" ON \"previous_events_events\".\"id\" = \"events\".\"previous_event_id\" INNER JOIN \"events\" \"next_events_events\" ON \"next_events_events\".\"id\" = \"events\".\"next_event_id\""

>> Event.joins(:previous_event, :next_event).where(previous_event: {name: 'prev'}, next_event: {name: 'next'})
#<ActiveRecord::Relation []>

>> Event.joins(:previous_event, :next_event).where(previous_event: {name: 'prev'}, next_event: {name: 'next event'})
#<ActiveRecord::Relation [#<Event id: 1, name: "current", created_at: "2015-02-11 12:38:38", updated_at: "2015-02-17 03:35:28", previous_event_id: 2, next_event_id: 6>]>

>> Event.joins(:previous_event, :next_event).where(previous_event: {name: 'prev'}, next_event: {name: 'next event'}).to_sql 
"SELECT \"events\".* FROM \"events\" INNER JOIN \"events\" \"previous_events_events\" ON \"previous_events_events\".\"id\" = \"events\".\"previous_event_id\" INNER JOIN \"events\" \"next_events_events\" ON \"next_events_events\".\"id\" = \"events\".\"next_event_id\" WHERE \"previous_events_events\".\"name\" = 'prev' AND \"next_events_events\".\"name\" = 'next event'"

>> Event.includes(:previous_event, :next_event).references(:previous_event, :next_event).where(previous_event: {name: 'prev'}, next_event: {name: 'next event'}).to_sql 
"SELECT \"events\".\"id\" AS t0_r0, \"events\".\"name\" AS t0_r1, \"events\".\"created_at\" AS t0_r2, \"events\".\"updated_at\" AS t0_r3, \"events\".\"previous_event_id\" AS t0_r4, \"events\".\"next_event_id\" AS t0_r5, \"previous_events_events\".\"id\" AS t1_r0, \"previous_events_events\".\"name\" AS t1_r1, \"previous_events_events\".\"created_at\" AS t1_r2, \"previous_events_events\".\"updated_at\" AS t1_r3, \"previous_events_events\".\"previous_event_id\" AS t1_r4, \"previous_events_events\".\"next_event_id\" AS t1_r5, \"next_events_events\".\"id\" AS t2_r0, \"next_events_events\".\"name\" AS t2_r1, \"next_events_events\".\"created_at\" AS t2_r2, \"next_events_events\".\"updated_at\" AS t2_r3, \"next_events_events\".\"previous_event_id\" AS t2_r4, \"next_events_events\".\"next_event_id\" AS t2_r5 FROM \"events\" LEFT OUTER JOIN \"events\" \"previous_events_events\" ON \"previous_events_events\".\"id\" = \"events\".\"previous_event_id\" LEFT OUTER JOIN \"events\" \"next_events_events\" ON \"next_events_events\".\"id\" = \"events\".\"next_event_id\" WHERE \"previous_events_events\".\"name\" = 'prev' AND \"next_events_events\".\"name\" = 'next event'"

>> Event.includes(:previous_event, :next_event).references(:previous_event, :next_event).where(previous_event: {name: 'prev'}, next_event: {name: 'next event'})
#<ActiveRecord::Relation [#<Event id: 1, name: "current", created_at: "2015-02-11 12:38:38", updated_at: "2015-02-17 03:35:28", previous_event_id: 2, next_event_id: 6>]>

>> Event.joins(:previous_events, :next_events)
#<ActiveRecord::Relation []>

>> Event.joins(:previous_events, :next_events).to_sql
"SELECT \"events\".* FROM \"events\" INNER JOIN \"events\" \"previous_events_events\" ON \"previous_events_events\".\"next_event_id\" = \"events\".\"id\" INNER JOIN \"events\" \"next_events_events\" ON \"next_events_events\".\"previous_event_id\" = \"events\".\"id\""

>> Event.includes(:previous_events, :next_events).references(:previous_events, :next_events)
#<ActiveRecord::Relation [#<Event id: 1, name: "current", created_at: "2015-02-11 12:38:38", updated_at: "2015-02-17 03:35:28", previous_event_id: 2, next_event_id: 6>, #<Event id: 2, name: "prev", created_at: "2015-02-11 12:40:10", updated_at: "2015-02-15 21:08:48", previous_event_id: nil, next_event_id: nil>, #<Event id: 3, name: nil, created_at: "2015-02-11 12:40:29", updated_at: "2015-02-11 12:40:29", previous_event_id: nil, next_event_id: nil>, #<Event id: 4, name: "hoge event", created_at: "2015-02-15 21:03:11", updated_at: "2015-02-15 21:03:11", previous_event_id: nil, next_event_id: nil>, #<Event id: 5, name: "prev event", created_at: "2015-02-15 21:03:32", updated_at: "2015-02-15 21:03:32", previous_event_id: nil, next_event_id: nil>, #<Event id: 6, name: "next event", created_at: "2015-02-15 21:03:44", updated_at: "2015-02-15 21:03:44", previous_event_id: nil, next_event_id: nil>]>

>> Event.includes(:previous_events, :next_events).references(:previous_events, :next_events).to_sql
"SELECT \"events\".\"id\" AS t0_r0, \"events\".\"name\" AS t0_r1, \"events\".\"created_at\" AS t0_r2, \"events\".\"updated_at\" AS t0_r3, \"events\".\"previous_event_id\" AS t0_r4, \"events\".\"next_event_id\" AS t0_r5, \"previous_events_events\".\"id\" AS t1_r0, \"previous_events_events\".\"name\" AS t1_r1, \"previous_events_events\".\"created_at\" AS t1_r2, \"previous_events_events\".\"updated_at\" AS t1_r3, \"previous_events_events\".\"previous_event_id\" AS t1_r4, \"previous_events_events\".\"next_event_id\" AS t1_r5, \"next_events_events\".\"id\" AS t2_r0, \"next_events_events\".\"name\" AS t2_r1, \"next_events_events\".\"created_at\" AS t2_r2, \"next_events_events\".\"updated_at\" AS t2_r3, \"next_events_events\".\"previous_event_id\" AS t2_r4, \"next_events_events\".\"next_event_id\" AS t2_r5 FROM \"events\" LEFT OUTER JOIN \"events\" \"previous_events_events\" ON \"previous_events_events\".\"next_event_id\" = \"events\".\"id\" LEFT OUTER JOIN \"events\" \"next_events_events\" ON \"next_events_events\".\"previous_event_id\" = \"events\".\"id\""