micropost.rb

class Micropost < ApplicationRecord
  belongs_to :user
  before_save { self.in_reply_to =  reply_user }
  default_scope -> { order(created_at: :desc) }
  mount_uploader :picture, PictureUploader
  validates :user_id, presence: true
  validates :content, presence: true, length: { maximum: 140 }
  validate :picture_size
  scope :including_replies, ->(user) { where("user_id = ? OR (user_id IN (SELECT followed_id FROM relationships WHERE follower_id = ?) AND (in_reply_to = ? OR in_reply_to LIKE ?))", user.id, user.id, "", "%@#{user.id}\-#{user.name.sub(/\s/,'-').downcase}%" ) }

  private
    def picture_size
      if picture.size > 5.megabytes
        errors.add(:picture, "should be less than 5MB")
      end
    end

    def reply_user
      reply_user = ""
      if content.match?(/(@[^\s]+)\s.*/)
        reply_users = content.scan(/@\d+\-[^\s]+\s/).map do |reply_user|
          reply_user.gsub(" ", "")
        end
        if reply_users.length == 1
          reply_user = reply_users[0].downcase
        else
          reply_user = reply_users.join(", ").downcase
        end
      end
      return reply_user
    end
end

micropost_a.rb

class Micropost < ApplicationRecord
  const MENTION_REGX = /@\d+\-[^\s]+\s/
  
  belongs_to :user
  before_save { self.in_reply_to =  reply_user }
  default_scope -> { order(created_at: :desc) }
  mount_uploader :picture, PictureUploader
  validates :user_id, presence: true
  validates :content, presence: true, length: { maximum: 140 }
  validate :picture_size
  # LIKE 文、便利なのですが INDEX が効かずにテーブルスキャン（全行検索）になってしまうので、できればテーブル設計の時点避けたい実装ですね :thinking_face: 
  # 特に今回のマイクロポストのようにユーザーが大量に文書を投稿するシステムの場合、この実装はしばしば致命的になります。
  scope :including_replies, ->(user) { where("user_id = ? OR (user_id IN (SELECT followed_id FROM relationships WHERE follower_id = ?) AND (in_reply_to = ? OR in_reply_to LIKE ?))", user.id, user.id, "", "%@#{user.id}\-#{user.name.sub(/\s/,'-').downcase}%" ) }

  private
    def picture_size
      if picture.size > 5.megabytes
        errors.add(:picture, "should be less than 5MB")
      end
    end

    def reply_user
      # ファストリターンを積極的に使用してインデントが深くならないような実装を心がけましょう
      # 繰り返し現れるリテラルは定数に置き換えたいですね
      return "" unless content.match?(Micropost::MENTION_REGX)

      reply_users = content.scan(Micropost::MENTION_REGX).map do |reply_user|
        reply_user.gsub(" ", "")
      end
      
      # ruby の if は if 文ではなく、if 式です。
      # if が値を返し、かつこの場合は真偽どちらでも reply_user が返っていたので代入が不要になります。
      if reply_users.length == 1
        reply_users[0].downcase
      else
        reply_users.join(", ").downcase
      end
      # ruby はメソッドの最後に評価された式の値が帰るので return は不要で、かつ上記 if 式が reply_user を返すので return reply_user は不要になります
    end
  end
end

micropost_b.rb

class Micropost < ApplicationRecord
  const MENTION_REGX = /@\d+\-[^\s]+\s/
  
  belongs_to :user
  before_save { self.in_reply_to =  reply_user }
  default_scope -> { order(created_at: :desc) }
  mount_uploader :picture, PictureUploader
  validates :user_id, presence: true
  validates :content, presence: true, length: { maximum: 140 }
  validate :picture_size
  scope :including_replies, ->(user) { where("user_id = ? OR (user_id IN (SELECT followed_id FROM relationships WHERE follower_id = ?) AND (in_reply_to = ? OR in_reply_to LIKE ?))", user.id, user.id, "", "%@#{user.id}\-#{user.name.sub(/\s/,'-').downcase}%" ) }

  private
    def picture_size
      if picture.size > 5.megabytes
        errors.add(:picture, "should be less than 5MB")
      end
    end

    # 空配列も join 可能ですし、正規表現を見直してこうかな
    # ex)
    # '@1-hogehoge @2-hugahuga message'.scan(/@\d+\-[^\s]+/).join(", ").downcase
    # => "@1-hogehoge, @2-hugahuga"      
    def reply_user
      return "" unless content.match?(Micropost::MENTION_REGX)
      content.scan(Micropost::MENTION_REGX).join(", ").downcase
    end
  end
end