class ActiveRecord::QueryMethods::WhereChain

# WHERE "authors"."id" IS NULL AND "comments"."id" IS NULL
# LEFT OUTER JOIN "comments" ON "comments"."post_id" = "posts"."id"
# LEFT OUTER JOIN "authors" ON "authors"."id" = "posts"."author_id"
# SELECT "posts".* FROM "posts"
Post.where.missing(:author, :comments)

that are missing both an author and any comments:
Additionally, multiple relations can be combined. This will return posts

# WHERE "authors"."id" IS NULL
# LEFT OUTER JOIN "authors" ON "authors"."id" = "posts"."author_id"
# SELECT "posts".* FROM "posts"
Post.where.missing(:author)

For example, posts that are missing a related author:

missing relations.
Returns a new relation with left outer joins and where clause to identify

def missing(*associations)
  associations.each do |association|
    reflection = scope_association_reflection(association)
    @scope.left_outer_joins!(association)
    if @scope.table_name == reflection.table_name
      @scope.where!(association => { reflection.association_primary_key => nil })
    else
      @scope.where!(reflection.table_name => { reflection.association_primary_key => nil })
    end
  end
  @scope
end