class ActiveRecord::QueryMethods::WhereChain

# WHERE "authors"."id" IS NOT NULL AND "comments"."id" IS NOT NULL
# INNER JOIN "comments" ON "comments"."post_id" = "posts"."id"
# INNER JOIN "authors" ON "authors"."id" = "posts"."author_id"
# SELECT "posts".* FROM "posts"
Post.where.associated(:author, :comments)

associated to both an author and any comments:
Additionally, multiple relations can be combined. This will return posts

# WHERE "authors"."id" IS NOT NULL
# INNER JOIN "authors" ON "authors"."id" = "posts"."author_id"
# SELECT "posts".* FROM "posts"
Post.where.associated(:author)

For example, posts that are associated to a related author:

associated relations.
Returns a new relation with joins and where clause to identify

def associated(*associations)
  associations.each do |association|
    reflection = scope_association_reflection(association)
    @scope.joins!(association)
    self.not(reflection.table_name => { reflection.association_primary_key => nil })
  end
  @scope
end