def self.combined_tasks(tag_ids)
    tasks = []
    tag_ids.each do |tag_id|
      tasks << self.find(tag_id).tasks
    end

    tasks.inject { |result, elem| result & elem }
end

def self.in_all_tags(tags_ids)
  tags = Tag.combined_tasks(tag_ids)
end

Task.joins(:tag_tasks).where(tag_tasks: {tag_id: tag_ids}).uniq