상황

class A
  belongs_to :B
end

class B
  has_one :A 
  has_one :C 

  scope :with_c, -> { joins(:c) }
  scope :without_c, -> { left_outer_joins(:c).merge(C.where(id: nil)) }
end

class C
  belong_to :B 
end

> B.with_c.to_sql
SELECT "bs".* FROM "bs" INNER JOIN "cs" ON "cs"."b_id" = "bs"."id"

> A.joins(:b).merge(B.with_c).to_sql
SELECT "as".* FROM "as" INNER JOIN "bs" ON "bs"."id" = "as"."b_id" LEFT OUTER JOIN "cs" ON "cs"."b_id" = "bs"."id"

> B.without_c.to_sql
SELECT "bs".* FROM "bs" INNER JOIN "cs" ON "cs"."b_id" = "bs"."id" WHERE "cs"."id" IS NULL

> A.joins(:b).merge(B.without_c).to_sql
ActiveRecord::ConfigurationError: Can't join 'A' to association named 'c'; perhaps you misspelled it?
from /path/to/activerecord-5.0.0/lib/active_record/associations/join_dependency.rb:231:in `find_reflection'

질문

지금 생각하고 있는 해결안

정답?

The solution is to build JoinAssociation when two relations with join information are being merged. And later while building the Arel use the previously built JoinAssociation record in JoinDependency#graft to build the right from clause. Fixes #3002.