OR

scope :tagged_any, ->(tags) {
  where(
    id: Tagging
          .select(:question_id)
          .where(tag_id: tags)
          .build_arel
  )
}

AND

scope :tagged_all, ->(tags) {
  tags.inject(self) {|scope, tag|
    scope.where(
      id: Tagging
            .select(:question_id)
            .where(tag_id: tag)
            .build_arel
    )
  }
}

SELECT "questions".* FROM "questions"  WHERE "questions"."id" IN (SELECT question_id FROM "taggings"  WHERE "taggings"."tag_id" = 1) AND "questions"."id" IN (SELECT question_id FROM "taggings"  WHERE "taggings"."tag_id" = 2) AND "questions"."id" IN (SELECT question_id FROM "taggings"  WHERE "taggings"."tag_id" = 3)

=> SELECT tag_list FROM questions LIMIT 3;

       tag_list
----------------------
 {android,googleplay}
 {ruby,rails}
 {java,android}
(3 rows)

OR

scope :tagged_any, ->(tags) {
  where(
    'tag_list::text[] && ARRAY[?]',
    tags.map(&:name)
  )
}

AND

scope :tagged_all, ->(tags) {
  where(
    'tag_list::text[] @> ARRAY[?]',
    tags.map(&:name)
  )
}

SELECT "questions".*
FROM "questions"
WHERE (tag_list::text[] @> ARRAY['tag1','tag2','tag3'])